I had a similar problem with my 1988 300 ce, If the car got to warm or it was hot out, I'd have to let it sit and it would start up fine. After much reading I found out if I unplugged the ovp and the fpr (& plugged them back in) behind the battery the car would fire right up without having to wait the 10 to 20 min for cool down. Then on a fluke I replaced the fuel pump relay first and of course that fixed the problem. All I can say is unplug the two of them next time the car won't start. If it starts back up you have narrowed it down to these 2 relays. Good luck