I recently inherited a 92' lumina (3.8) with 8,000 miles on a rebuilt transmission (out of warranty period). My problem is a shutter/slipping when momentarily slowing down from cruising speed (above 45 mph) and accelerating again. The trans. downshifts from 4th gear (overdrive) to 3rd gear. Once in third gear and I accelerate to regain cruising speed, the transmission has a slight (but noticeable) shutter and hesitation. If I accelerate hard to downshift to 2nd it stops. If I accelerate "slightly" to regain speed the shuttering only last a second or two and then sort of "kicks" in and shifts to 4th with no problems. Anyone have any ideas? I've drained some fluid out and added LUCAS transmission additive in hopes of a quick fix but it hasn't helped.