Since the car won't restart sometimes when it acts up, you have an opportunity there to do a simple test while you're problem is occurring but you will need to be prepared.

One of the simple tests I do on all the engines I work on when they quit, (lawnmowers, chainsaws, motorcycles, cars...) is a gas starvation test.
I use a small gas can or use a turkey baster for some sidways carburators to give the engine a little shot of gas when it won't start. IF it kicks over, but then dies when it burns up that shot of gas, then I know the problem is fuel delivery related and I don't waste time chasing down ignition related problems. I go after the fuel delivery system components.

My 560 did something similar to what you discribe a few years ago. I would drive somewhere and turn it off to go into a store, then when I went to leave, it wouldn't restart for an hour. I ended up carrying a small gas can (which was smelly) and traced it to fuel delivery issue by the method I described above.

Luckily for me, someone locally was parting out a 560 and I salvaged their whole injector system and rebuilt my whole fuel injection system and replaced the fuel pump relay and it's never been an issue since. I am not sure what exactly the issue was, but I know it was a fuel delivery issue.

If I was you, I would find another working fuel pump relay to carry around and next time the car "boinks" on you, swap the fuel pump relay with the extra one.