My 86 IROC has the same early shift problem as some of the others in this forum except for 1 important difference. If I stand on it , it works perfectly, gets tire at each shift. If I start out normally, shifts way early and seems to engage lock-up early too. I have a shift kit, new speedo (generic), new tv cable, adjusted many times. One other thing, since the shift kit 2 years ago, it has always shifted hard from P to R, by that I mean it launches into gear under normal conditions. Since the new problem, only sometimes launches. Any ideas where to look next?

 

did you swap rear gears?

 

No, no changes there.