I have a 91 chevy 3/4 ton with a 6.2 and 4L80E tranny. The trucks tranny spartically acts up and acts like the tranny is slipping, usually when it's warm. It doesn't do this all the time so the tranny can't be bad. I've replaced the ignition switch and checked the output speed sensor, and neither have fixed the problem any ideas what to do next? It seems like bumps cause it to act up more than smooth roads????  

first, start simple. have the filter changed and the tranny fluid flushed, not just emptied and replaced like your engine oil. then start chasing your wires going to the speed sensor. if the connector at the sensor is even slightly loose, it can create a problem. lastly, be certain the sensor itself is seated well