1990 Jaguar XJS Engine cranks, but doesn't fire.

Mostly on very hot, (Florida), days, after driving 10 - 20 miles, stop & Start, running errands the car is parked for a few hours.
When starting again, the engine turns over nicely, but never starts.
After about 12 - 20 hours just sitting, the engine starts nicely; and the problem is gone. Until the next time).
IO can never get the car to the mechanic before it starts runniung again- so he has nothing to look at.

HELP!

Trust me. If somebody provides a solution that works, I will contribute $50.00.