Comrades....
I have a 1989 firebird 305 with the automatic on the floor. It would not shift at the proper times so took it in to the tranny shop and they replaced the governor. It now shifts at the proper times but when I run it in normal drive 3rd it starts to shutter the whole car between 45 mph to 55 mph and continues. In overdrive it shutters at the same speed 45 to 50 mph and rpms go up to 2000 and then it shifts into 4th and runs like a baby. Real smooth. If I drop it back into 3rd it shutters with rpms going up. Back into overdrive and rpms go down to around 1300 and it runs smooth. Any ideas what may be causing the shutter?????

 

Sounds like torque converter shudder. If it only does it while locking and unlocking the converter then I would say the clutch mechanism in the converter is going south. MY two cents.

 

I agree with your two cents.