sluggish 736, but only on start up for the day

The fact that this happens only when first running it, and the problem doesn't happen after running it, makes me think a couple of things:

1)What kind of grease are you using on the gears? Some non hobby grease when it is cold can be really sticky, and that would fit your pattern, once the engine breaks free so to speak, the grease heats up as the gears and engine heat up, and it runs fine. I would when it was cold (hadn't been run), pull the brush plate and take out the armeture and check the grease on the end of it, if it is somewhat hard and tacky/sticky, not easily pliable, you may want to switch to a hobby grease.  Also check the spiral on the end of the armature and check the gear teeth it meshes with, examine them for missing teeth or broken teeth, it could cause problems until it starts working, though that wouldn't explain why it worked when it was warm. If you can stop and start the engine every time once it is warm and it is fine, not likely to be the gearing, not to mention I would expect it would work sometime when cold...it is why I suspect the grease. 

2)Potentially it could be that the commutator on the armature is dirty, and it isn't allowing the engine to create enough power to move, but once it starts moving the brushes are able to make enough connection electrically to run.  I would clean the commutator and the brushes just in case that is the cause (doesn't hurt, anyway). 

With stiff grease you might be able to turn the wheels easily when it is cold, but if you tried to manually turn the armature to turn the wheels it would be hard. I would recommend cleaning out the grease used in the gearbox, and replace it with new grease designed for engines, and see what happens, certainly not going to hurt and only cost is time and a bit of grease. 

I would follow the above advice and completely clean out the grease in the gear box and replace it. Red N Tacky is a favorite on the forum. Labelle 106 is a hobby grease that could be used as well.

I use Labelle 106 Hobby grease.

I thought I read a discussion here about excessive Red N'Tacky causing locos to be sluggish.
Maybe you used too much.
As BigKid and Bmoran4 wrote, perhaps you have old grease that needs to be cleaned out.
On the first service of a used locomotive, I usually clean all the old grease, lint, and dirt out of the gear's teeth (worm and worm wheel on a Berk)

It sounds like you have carbon buildup between the motor's armature coils. This comes from the brushes over time. Take a toothpick and scrape the carbon out of the lines between the armature brush contact areas and spray with a bit of contact cleaner and that should help.

I used red n tacky. Cleaned out old grease first. ( which really wasn't hard, but did it anyway)   Oiled armature etc. All the usual stuff. I'm no Burfle, but these are not new to me. Done many of them. The engine isn't getting actual "cold" to where lube thickens. It's in the living room on the floor. I think I'll check armature/brushes again. Really nice engine. Thanks for the responses.