Ocasionally tugging while cruising..

Hey guys, what might be causing my 1998 Monte with a 3.1 to tug slightly while cruising? It doesn't always do it, but after the TCC locks, it will start tugging, kind of fishbite-ish and you can see the tachometer bobbing up and down about 100 RPMs at a rate of a copule times a second. It's been tuned up and runs great otherwise. Thanks in advance.

This can be caused by a slight misfire when the engine is loaded by tcc engagement, or the tcc is slipping, a full scan would should slippage percentages in tcc lock up.

I agree with Max......it can be a misfire(even though you had a tuneup)....Possibly a coil/ICM problem.....or......

Could be a tranny slip......