I have a 91 chevy s-10. 4.3L Automatic. I recently took the truck in for diagnosis. The timing is ok. Oil pressure ok. Compression...ect. The mechanic found absolutely nothing wrong. Under small load at medium speeds or uphill, the truck jerks noticeably. Feels like a mis-fire, or a hiccup. It does it in OverDrive but not in Drive. I am afraid it is transmission...What can I do to test this theory without petentially having a break down? I don't want to take it back and spend another 50 dollars for them to tell me it is nothing.

My guess would be the lockup torque convertor is causing your hiccup. To improve efficiency, the torque convertor is locked when in Over-drive and under light load. This effectively couples the engine to the transmission and does not waste HP by heating up fluid in the convertor. When you are on the gas, the convertor will try to unlock and that is probably when you feel the hiccups. I had a Corvette with a similar problem and just kinda lived with it for a while. The answer is a new torque convertor.