This doesn't sound like a fuel pump relay problem to me. Normally, when the relay fails, it does so when you are trying to start the car--even in cases where the relay is intermittent. I've not seen a case where a failing fuel pump relay would induce an engine miss under load, but I guess it's possible.
I would look closely at the ignition system, especially the spark plug wires and the coil. Had a coil go bad recently on my 560 engine--it's not a cheap item.