Help! - I'm stumped with this one!

Forgot to mention:

     The smoke unit is off.

     The problem occurs whether Odyssey is on or off, but it is worse with the Odyssey on.

Have you tried running the train on a different layout, to confirm that it is not a track or other problem that could be specific to the wiring of the layout?

Is the legacy base turned completely off? No power? Even if it’s not on the legacy track, even in another room, the legacy signal will find it.

When I run it in conventional, the Legacy base is turned off.  It is plugged in but there is no power to it.

As far as running it on another layout, he brought it to me because he said it was "taking off like a run-away train".  I assume that was because he was giving it a lot of power to make it through the stall period.

I'd be looking at the trucks and the pickup wiring, it's common for a turn in one direction to point out a broken/loose wire.

I'm in the John camp. Be sure all pick up rollers are doing the job. Lionel generally uses grommets in the frame where the wires come through the frame from the trucks, so a pick-up wire rubbing the frame probably not the problem. It sounds like a brief short to make it stall, then restart it's self.  What is baffling is it stalls only in conventional mode, but turns fine in command mode? You friend said it takes off full speed when it happens. It sounds like he was running in command mode, then a brief short happens and the engine drops into conv. mode. I would look for a mis-alighned roller assembly that is brushing the outside rail on the curves.

I came a cross a situation where a wire with an almost undetectable compromise in the insulation contacted the motor housing on a curve and shorted and stopped the engine. That was MTH, but it is something to consider, especially if it is in a particular direction as you stated.

I will look into it, but it is happening on the straights, as well.