Sounds like poor DCS signal strength. Most likely from just dirty track and/or wheels.
Magnified by other things like possible low batteries in the engines? So when they encounter quick losses of power they stall more.
I always suggest taking everything off the rails that draws power. Sometimes it's easier to use a spare piece of track.
DCS engines that move and then shutdown unexpectedly usually are just losing power. Possibly shorting but not so many?
Try running an engine without the TIU and see if it makes it around your loop. If it doesn't, this is not about DCS at all.
One more thing, Are your Lionel TMCC/Legacy engines on the rails? Tried disconnecting their base from the layout?