Random Engine stutter

Look at the underside of the loco.  If you see any blackened, or "bubbled" areas on the rollers or wheels, it indicates that there was heavy sparking or arcing, most likely a short circut.  Long-term, this will cause pitting and oxidation so it's worth a little trouble to figure it out.

You might use an ohmmeter to verify that both center rail rollers are connected to each other, picking up electricity, and not shorted to ground.  Also, that the rollers are properly insulated from the truck block, all screws are snug, and no loose or broken wires are causing short circuits.

Wild guess, but do both electrocouplers work as they should?

Guessing it has rubber tired wheels on the outer axles of the trucks and dummy wheel sets on the inner axles so for a split second it is losing the common connection as only the center wheel sets are all metal with good electical contact. If that is an MTH switch, MTH track doesn't connect the outer rails together like tubular tracks does.

Ted S

This engine tracks fine in every other turnout-just not these two. 
And no other engine has this issue. 

I will check the rollers for continuity. 

What is the vintage of the two problem switches?  I had an early (circa 1996-97) MTH RealTrax O72 remote switch as part of a floor layout that did the same thing, except with all of my locos, not just one.  I don't believe I ever identified the cause of the problem.  After a while the rails became heavily pitted from all the sparking.  That switch is in a landfill now.  I might have gotten $30 or $35 for it on the 'Bay, but I didn't want to pass the frustration on to anyone else in the hobby!!

Some other things to check:  Are the problem switches LEVEL (in both the direction of travel, and side-to-side across the rails?)  Is the switch's base plate "floating," or under torsion because it was fastened down too tightly to the platform surface?  Do the swivel rails have excessive play compared to your other RealTrax switches?  Something about these two is different, you just have to find it.  It's usually a lot of trouble to remove and replace switches on a permanent layout.  But swapping a couple around would be another way to experiment.

The SD70 is a long loco.  If it follows usual MTH practice the innermost axle is a "dummy" with small, unflanged wheels.  At one point i think these wheels were even made of plastic (which is a good thing, to prevent short circuits!)   Personally I've never been a fan of the dummy inside axle for performance or appearance.  With this design, the functional truck wheelbase defined by the outer two axles is very short.  This makes the trucks prone to weaving and bouncing.  So I'm not surprised that this loco is picky while your Geeps, F-units, etc., negotiate these switches uneventfully.

One last thing to check... is the track leading INTO the switch aligned straight, or does the loco enter the switch at an angle?  I tried to watch your video again but it's a little hard to follow.  IMO using your phone in landscape (horizontal) format works better for this sort of thing.  Keep troubleshooting and post your results!

Ted S

sorry for the Crappy video😁

one correction-

the turnout at the entrance to the reversing loop-is diverging  

the other turnout is not  

that adds a wrench to the monkey  

if you watch as the train leaves the reverse loop, it passes another 6 axel Diesel. That one has zero issues on any of those MTH turnouts. 

I’m convinced, by the evidence thus far

1) no other engines I own have this issue

2) that the issue is with the engine, not the track. 
But, time will tell. 

I did some investigating; neither the rollers or the track are pitted  

I found that one roller was not as proud as the other, and that it was askew. 

so I corrected those issues. 

the first attempt, ran flawlessly. 
I thought I had it solved. 

But, subsequent runs, had the same stutter. 

So I pulled the shell. 

Both trucks/ rollers are connected 

I see no damage/ faults that are apparent. 

There is a NI-CD 2.4V battery

which I believe needs upgraded  

I got this 30-2876-1 and a 20-2310-1

for a little more than what the GP-40 cost new, so the aggravation is offset  

I’ll post new developments as they occur

thanks

A new development 

I replaced the battery with a BCR2

A worthwhile upgrade that didn’t resolve the problem. 

so I got out my meter. 
the divergent route had 6 vac on the inside rail. 
so, I noticed my MTH Lock On  to that section only applied power on the outside rail. 

So I added a Lock On to feed the inside rails. 

Funny how I had a problem that I did not know I had. 

But that did not resolve the issue

So, next step

remove/ test- then-repair or replace the turn outs. 

And to those who might be inclined to opine

“You used the glue on purpose”