Check fuel pressure, if its good, stick a spare coil on and see if goes away, have injectors flowed/cleaned BUT they rarely act up hot, if still screwed up, buy a DUI or accell distributor module (part#9), they are as good or better than the merc at less cost plus your local autozone/advance auto prob has one on the shelf. Application is a 1988 to 1994 tbi Chevy truck w 5.7. Do NOT buy a standard or off brand brand module (like a duralast, JUNK). This spring I cleaned and flowed two sets of 500 efi injectors for a Michigan guy doing fresh builds, one ran similar to yours except it even broke up as soon as you put it under a load. He swapped ecms, coils, sensors etc motor to motor. In the end he changed that distributor module and problem disappeared . Ive seen them get hot and break up on my dyno too. The merc one has a # like 399 on it, almost everyone I seen screwed up was a cheap generic auto parts store one fwiw, Smitty

My guess is bad fuel pressure regulator...

Good to know SMitty. I have been around 1000's vehicle wise and usually OE GM modules create no spark when an issue. Have not run into higher rpm miss.
Will put that into old gray matter.

Oy, yes, re-read, tons of issues with aftermarket one's...all sorts of different issues, including cutting out at 3k+ rpm..... NAPA/Crapa, Durajunk, Standard Schit, BWD - big whoopy doo, etc, doh...

Thank you!!

Could be, its just odd that it runs great for almost an exact amount of time or until it reaches a temp and then fails.

Kinda but not really weird per say. Common for electrical, especially ignition components that create heat like modules and coils. If you change a module use dedicated heat transfer compound like arctic circle or etc, and applied properly. Electric Fuel pumps can do this too.ie: run, get hot, not work. Then cool it down, run okay for a bit again.

What about the fuel cooler?

I had a similar issue with backfiring that got progressively worse. It was a bad fuel pressure regular.

Thanks for the info.

You mentioned taking the gas cap off for a bit helped. Perhaps your tank is not venting and as you draw down fuel the tank vacuum is starving the motor and you are getting a lean pop.