Dave,
Welcome to the Forum!
I have a 1946 ~ Lionel 221 & 221W tender, so I  am familiar with some of it's quirks.

It sounds as if the roller pickup on the locomotive may be bent down too far and catching on the activator of the
uncoupling track as it passes over, causing the loco to jump/derail.
or
The actuator on the uncoupling track may be bent up enough to catch the roller pickup on the loco as it passes over.

If you have another uncoupling track, try replacing the  piece that you're having issues with and see it the loco still jumps/derails there. 

If that doesn't do the trick, then check the pickup roller.

Dave