I think the mechanic might be wrong. Before you spend a wad of money on a new tranny,check a few things out. Check the TPS with an ohmeter & spacers between the throttle stop and throttle adjustment screw. Also check tour timing. I think factory specs is 10 degrees before tdc. Try retarding 2 to 5 degrees. Let it warm up real good and set your rpm's at 850 to 900. One last thing, run some 93 octane in it. My truck was doing the same thing. I almost bought a tranny. I did a little tinkering, now it runs great. Hate to see you spend a lot of money needlessy. Good luck. Dave