My 1991 Mustang GT spudders, and hesitates when I start it up and while Im driving. It will sometimes die on my and I let it sit for 15 mins and it will fire right back up like nothing happened. It doesn't happen everytime through. It can run for weeks and not mess up then the next day it can be acting up. Im not sure what is wrong and the guy that works on my car can't figure it out. I took to Ford and they couldn't find anyting wrong with the car. Can anyone help me?