1984 Chevy Corvette ROUGH

I have a 1984 Corvette that has not been driven much. Yes 9100 original miles total. The car is running very rough. A few years ago I put some gas line antifreeze in the gas tank and it has never been the same as a matter of fact it has gotten worse. I since drained the tank and refilled it with fresh gas changed the gas filter. Gave it a full tuneup and it runs exactly the same. I took it to a local mechanic, he kept the car three weeks and said he gives up. At this point I have not even picked up the car yet, not sure where to look first. I can and will replace or test anything that you say. It's really ashame that a car with this mileage runs this poorly. HELP. Cant find someone to work on this car!