As Mike CT suggests above it could be contact with the outer rails as well as the center rail and pickups. It is not surprising that replacing the switch with a straight section of track fixed the problem. That could be another indication of something going on between the switch and those specific engines?
If you can specifically define the problem and describe it here, someone may be able to provide a solution. There are an assortment of folks here that between them have at one time or another experienced most every problem you can imagine. Lots of knowledgeable folks here!
Also, as someone suggested earlier, try holding your hand over the engine when it stops. I have seen this mentioned several times around here and I believe it indicates a poor signal or antenna problem? If something (or nothing) happens, I will again defer to the more knowledgeable here. I have not experienced this problem myself. Although I have a legacy system and a couple of Lionel engines, I am not too well versed on the TMCC/Legacy stuff or their problems.
