trany problem?
 

I have a 93 Corvette with an AT, totally stock. Out of the blue the car starts free wheeling if I let off the gas and either down shift or touch the brake pedal. This happens in all four gears. 1st, 2nd, D, and OD. I've taken it to a well known trans shop and a couple of hundred bucks later I'm told they have never come across this problem before. Usually the coast clutch only goes out on one of the gears not all four at once. Their advice, drive it like it is or a total tear down. I'm thinking this might be an activation switch somewhere. Any advice would be appreciated.
Chuck