My laziness has come back to bite me!

Start with the easiest.  Check that the connection to the transformer is tight on the posts that lead to the outer track.

Earl

Do you have trains stopping on the outer loop? If so, see if that coincides with crossing over a track joint. If your voltage is dropping as opposed to a complete loss of power your description of you layout sounds like you may be exceeding your transformers power output. How about some more detailed info?,i.e. the overall size of your layout, transformer type,are you running command or conventional, etc.

You may be correct, Earl. A few years ago I had this happen. I took out the transformer to that loop, it tested OK, put it back in, and everything worked. A prayer to Saint Volt may help, also.

If you decide to rewire your layout, here is how I wired my layout and it is very easy to troubleshoot, even though the layout wiring is messy and the wires are not color coded. 

I power my layout with a ZW transformer.  I have eight bare bus wires running the length of the layout.  Each bus wire has a jumper wire to one of the eight posts on the ZW.  I have a 9th bare bus wire running the length of the layout for the lights on the layout.  The power to the 9th wire is controlled by a single switch on the layout to turn the lights on and off.  I used copper house wiring, stripped of all insulation for the bus wires.

I have two loops of track.  All track connections connected to either the A-AU pair or D-DU pair of bus wires.

All control panel accessories go to studs under the layout that are electrically connected to control panel switches that connect the studs to ground to turn the accessories on and off.

All accessories are either connected to the B bus wire or the C bus wire dependent on their voltage requirements and the other connection is to one of the switch controlled ground studs.  I set the C channel of the ZW to around 12-14 volts which is the normal accessory voltage requirement.  I set the B channel to a higher voltage to control the turnouts and the uncoupling magnets.

All connections to the bus wires and studs are made with red or black alligator clips.  All red alligator clips connect to the hot bus wires (A B C D) and all black alligator clips connect to the ground bus wires (AU BU CU DU).  So if, for example, I have a short on one of the ZW channels, I just go under the layout and remove all the red alligator clips and start re-connecting them, one at a time, until the short is found or I can just disconnect them, one at a time, until the short is found.

It is also very easy to either change out accessories or move them because all I have to do is remove two alligator clips to remove the accessory or install two alligator clips to install the accessory.

I have not had any problems with using alligator clips for connections with either DCS, TMCC, or Legacy.  I have had this layout for about 12 years and troubleshooting any wiring problems that have come up has been a breeze.

Hope this helps...

Earl

Both Lee and Earl have a lot of wisdom wrapped up in their replies, completely different though their approaches are.  And of course you should start with the easiest thing first.

One other thing though.  Did you make any changes just before the problem started?  Thinking this through has saved me more than once.

My wiring is a rat's nest like Lee's.  To answer your question, "what would you do?", I'd just run a new wire too.  Doing that will tell you for sure if the wiring is the problem or not.  Clip out the old wire if you don't want to add to the confusion.

I had only 2 wires going to my whole 11x12 layout for over a year!

Now I have 4.  2 for the DCS system and 2 for the transformer powering my building LEDs.  I FINALLY hooked up my small, 7-building downtown area using LEDs, 1 per floor in each building.

As long as it keeps working I don't see any more wiring for me.  All my switches/turnouts are hand-thrown using Caboose Industry Ground Throws.  About the only thing I MAY do is install 2 On/Off switches so I can leave PS2 and Conventional engines on the track.

You guys that like a bunch of animated things on your layout are nutz , although I am starting to look at the Miller signs for one that would look good downtown.

Who knows, before it's over with my wife might find me entangled in a ball of wires, lit up like a Christmas tree

Well, Lee, it takes real guts for a licensed professional engineer to admit this.  

It is an 8x8' loop with one 4'x4' passing siding and a shorter spur. It has one passenger train w/ three lighted cars and one lighted dummy on the siding. There are three 0-36 Fastrack switches in this loop.  It is powered by its own Z-1000 and run in conventional mode. At times the train will run full speed. Sometimes it slows down or even comes to a stop. The headlight on the dummy engine on the passing siding will shine with brightness or a lack thereof relative to the speed of the engine on the mainline. Fast = bright, Slow = dull; Stopped = off.

Well, I freely admit it - my trains are an escape from the computers (hence I run only conventional) and the discipline (hence the rat's nests) of work, something to escape to, or "de-compress," as I call it, after a day at work. 

I have to admit that I've never really been stung by this approach - such as having  a loop that defied easy fixes by running another wire.

OK, I design Electronics for a living these days.

My Layout has been running for several years now with Zero Blocks (except sidings), 2 Power drops on opposing ends, and 5 single ended sidings powered thru bat wing toggle switches that have stayed on all the time.

No hassle, no issues. DCS even works fine.

I do have a plan to upgrade and the wiring is getting upgraded at the same time. When I can afford the lumber and track...

Anyway, I'd snip out the bad wire when I add a new one since DCS doesn't like doubles, Since you run Conventional, add a wire, no issues.

Russell, you brought up a good point.  Things are very different.  It I were running DCC systems of any type I would have had to practice more discipline in both planning, and layout out the wiring.  I am not sure my layout would even support a digital control system right now without a lot of re-wiring of the feeds to the track. 

Came to O from N scale where DCC is very much alive and well. I understand all to well how the spaghetti under the table can get out of hand and the reason for my advice in my previous post. I guess that's why the transition to DCS-TMCC/Legacy wasn't much of a problem.

David