Why does my S10 Blazer shift so hard when in Overdrive but not when in Drive?

1985 Chevy S10 Blazer 2.8L Automatic 4WD - Have transmission work done three times in 1 1/2 years. Repairman says govenor keeps breaking (whatever that means) and will happen again. Shifts hard going into 3rd gear only when in Overdrive position, does not do it in Drive position. At 50mph or higher can slip from D to OD and shifts fine into OD. What's going on with my truck?