tranny problems

I've noticed that sometimes when I hit a hard bump my transmission will slip out of gear for a brief moment. Kinda like throwing it in neutral and back in drive. The rpm's will rev up until re-engaged. I have an 88 XJ 4.0 with auto. No tranny modifications. I'm curious about what is causing it. Any ideas?

Weird... could it be that the torque converter is unlocking? Maybe a loose/bad connection to the tranny? Loose brake pedal? The TC will unlock if you hit the brakes. Weird.

Dan