I recently purchased an '85 with 82K miles on it with the 4+3 Nash transmission. The overdrive worked well when I purchased it, but recently it will not stay in overdrive. My mechanic says the switch is ok and he can get it to go into overdrive, by letting off on the gas and letting the RPMs get below 1k RPMs, but when he steps on the pedal, the overdrive does not seem to be engaged and the RPMs return to the level of pre-overdrive.

I was having trouble getting it to idle at below 1000 RPMs about the same time this problem surfaced and he checked and fixed an apparent problem with that, but it still will not stay in OD.

Any help would be greatly appreciated.  

Hello
The early Corvette 4+3 trans have a cable from the throttle body to the trans, like a auto trans, If it's out of adjustment the trans will not shift correctly.
David Steele