Not sure from your description exactly what it's doing, but hopefully this might help.
I chased this through the whole ignition and fuel system for a LONG time.
It turned out to be simple, the fuel was getting overheated.
When I finally ran a vacuum/fuel pressure gauge out through my scoop into the cabin, I watched my fuel pressure drop to zero.
I heat sheathed the 3/8" supply line by the headers and ran a standard 5/16" fuel line for a return.
One or both fixed it.
I have been hearing a lot lately about people having problems with fuel boiling/evaporating/vapor locking, getting too hot in the lines from proximity to the headers/exhaust, and possibly in the pump as well. New fuel SUCKS! It cannot take any heat
Good luck!
You can watch the tach hit 4k and then drop. After I lifted it came right back and cruised at 3k (70mph) no problem.
This is on the same stretch of road, after the fix.
