911E - Engine Cutting Out after warming up

Hi all. I have a '71 911E that I restored a few years ago. The car ran perfectly for over 2000 miles. Recently I went on a 1 hour drive and it ran fine but started to stumble a bit after I got off the highway.
It runs fine for 30 minutes or so, stumbles a bit , then completely dies. So I pull over, turn off the key for 5-10 seconds, start it again and it runs perfectly for a while - then same thing happens.
I discovered that if I turn off the key for a 5 seconds or so when it's stumbling and I coast along in gear, and then turn the key it back on, it would run fine for a while. It's as if something electrical is overloading/overheating or something, and killing the power for a moment seems to reset it.
The MFI was redone by Gus Pfister. Bosch CDI box and RPM Transducer recently redone by Ashlock. I put in a new Parts Klassic coil. I even replace the fuel filter for good measure. Still the same problem. What should I check/test/replace next? Goin' crazy here....
Thanks.

Do you still have the original coil?

I don't. It wasn't correct when I got the car years ago. So I bought an original black Bosch coil when I restored the car. I recently tried a Parts Klassic coil but I'm still having problems with the motor cutting out as I described above.

Possibly an overheating fuel pump?

Swap your coil back to the old Bosch unit you have as a test. Hook up a timing light and see if you've lost spark when it dies. If spark is still good then consider inspecting the in tank filter.

Thanks for the input! I'll check out the the spark and filter.

Check your fuel pump pressure.

I will. Pump is fairly new - 2000 miles only. But you never know!
Thanks.