Why does my n gauge engine jump off the track sometimes. And why do some cars disconnect when it's moving?

An N gauge engine may jump the track for a number of different reasons, so you will have to do some careful analysis of the occurrence before you can find a solution. Joints in the track may be uneven, wheels on the engine may be out of alignment or out of gauge, the wheels or axles may be too loose or the leading truck has too much (or not enough) flexibility, or a number of other things. Sorry I can't give a simple answer without observing the problem.

Similar answers for the uncoupling of cars in motion. Uneven track or joints, misaligned coupler height, out of gauge wheels, loose trucks or axles, and much more. Again, it will take some careful observation to determine the cause before you find a cure.

Once you determine the cause, be sure to ask here for solutions if you need to. Model RR is fun--if your equipment is operating as you expect. There are plenty of sources for help with all aspects of model railroading.