99% sure it's a sticking solenoid. Exactly the same symptoms and problem as my old XR3i used to have. It used to be very embarassing when i'd ran it warm for a while then went to get petrol so turned the engine off, got back in the car and it wouldn't start. If you haven't fixed it yet, next time it does it try giving the solenoid a firm tap with something solid as this usually frees it. This is not a cure but it might get you out of some red faced situations.

Let us know what it turns out to be.