Why wont my corvette stay in overdrive?

I have a 1985 corvette 5.7 350 tune port that wont stay into overdrive. When im cruising at about 60 in 4th gear(its a 4 sp.) im at 3 grand and when i let off the gas the overdrive kicks in, then when I even lightly touch the gas, the overdrive kicks out and im back at 3 grand deccelerating. I dont get, just lightly give it gas. The corvette has a 4+3 doug nash and the O/D wont stay on.