I recently purchased a '99 S10 4x4 pickup. Soon after my purchase, the tachometer started jumping a few hundred RPMs, or sometimes less, when the automatic transmission had shifted into 4th gear. It's a 4-speed. I tried putting it in 3rd, and the problem is much less pronounced, but still exists. The revving can easily be felt, as it causes the vehicle to lurch ever so slightly. The transmission had plenty of fluid when I bought it, and I have noticed no leaks. Not sure when the last time it was replaced was, though. The problem is far more pronounced once the engine has had time to heat up. The transmission seems to shift into gears fine, right when it is supposed to. I'm wondering if this could be a problem with the torque converter, maybe the lock-up clutch? Or could a band for 4th gear be slipping in the transmission itself? I've also heard that dirty fuel injectors, vaccum leaks, and myriad other, much less serious problems can cause similar symptoms. Also, there is no shudder when I accelerate, as many with a torque converter problem have.

Any thoughts as to what might cause this problem? If I don't fix it, is it likely to kill the whole transmission in the near future or something terrible like that? I don't have the money for a complete overhaul... Any thoughts at all would be helpful!