hmm...i had that same problem in my V6, its still prevalent but it doesn't jerk and chirp the wheels as it used too. It used to happen as i accelerated from 1st, then let the gas off before it hit 2nd gear. Apparently, the throttle would hang at 3000-4000 rpm, then rev up as the transmission went to neutral and tried to engage second and then "POP! SQUEEK! BAM!" as my head lurched forward, as i said "damn, what the hell was that?". I've had my transmissoin fluid flushed, and i've changed my acceleration habits so i don't stop giving it gas till it goes through second gear (usualy it shifts at 2500 rpm in neighborhoods) so its not near as bad as when i first got the car.
Silver 1998 Mystique LS (v6)...ATX

Removed Emblems, limo tint, painted calipers.
Premier P630 HU with 4 Pioneer TS-A6895 Speakers with 10' Sony sub (yeah i know, i'm cheap)
Commando 501-S Alarm