i should also add that the prblems only occurs under load. it always revs freely to the redline

 

Sounds like a classic case of bad oxygen sensor(s). They are essentially off during the engines warm-up period and only start to regulate computer functions after you are up to running temperature. A bad O2 sensor will really screw everything up and give you crap for gas mileage as well. I am not too familiar with the '88 GT engine but I would think it is still way new enough to have an O2 sensor of some kind. They don't last forever and are very important for good running. Good luck!