Check the trucks and make sure that you don't feel any bumps or ridges when you turn them by hand. Push them up against the floor when you turn them and it should be perfectly smooth. If you feel the slightest irregularity, post and I will give you directions on where to look w/photos. This is a problem I had that would give intermittant derails, sometimes depending on direction of car; I found the problem and haven't had a derail since.
Make sure the diaphrams are not snagging each other. I also had this problem with the lightweights. If this is the case, you will need to replace the couplers with those that have longer shanks.