why does my 89 gt 5.o not start when hot?
Most common is the ignition module then the distributor pickup coil. Then there is the fuel system, it could be vapor locking. So you really need to find out if you have spark and fuel pressure and flow when it is in failure.
Like tennis shoes mentioned , I'm also guessing it's your TFI Module on the side of the distributor. It will give you the no start when hot problem.
Looking for a Used Mustang in your area?
CarGurus has 38,571 nationwide Mustang listings starting at $1,963.
Search Ford Mustang Questions
Ford Mustang Experts