It only slips in drive,not in L1 or L2??.I've had a few TH350's that acted like that when the modulator valve's vaccuum hose came off(bad valve can do the same also).If it feels like the tranny goes into neutral when you let off the gas in drive,and coasts freely,then feel like it goes back into drive when you give it more throttle,this may be the cause.Check the hose on the modulator,if it still on,check for vaccuum at the hose with engine running in park.If you are getting vaccuum there,check the modulator for any evidence of dextron coming out of the nipple for the vaccuum hose(a small lenth of string works,or you can just remove the modulator,one bolt with a clip holds it in)and tip it,if dextron leaks out the modulator's diaphram is shot,you need a new modulator.
Most of the TH350's I had that died usually lose reverse first,slipping at first,then no reverse,followed by no foreward gears a short time later.A hundred other things could cause your problem too,like a clogged filter,water in the fluid(possibly from condensation from sitting,causing valve body or pressure regulator to get rusty and stick),worn clutches,loose valve body bolts--the modulator is the easiest to change and the first thing to check.Good Luck! /forums/images/graemlins/smile.gifP.S. Make sure the shift linkage is properly adjusted also!.
