[Old Guy with 87 GT]

if it's not the relay sticking then check out the distributer ....on the ford system the computer controls the relay depending on what signals it gets
......when it gets a signal from the ignition it only turns on the pump for a couple of seconds to pressuerize system ,
......after that it doesn't turn the pump on constant flow till it gets a signal from the distributer(telling it the cars running )....there is a pick up in the distributer that sences rotation (its a seperate circuit in the tfi )if the pick up in the distibuter fails it can fool the computer into thinking the car is still running

my 87 did this a couple of times .....i cleaned the distributer out (cotact cleaner and air)and cleaned the contacts for the tfi and it hasn't done it since