I have a 97 classic wagon .50,000 miles which is standard except for a racebox back box.

It persists in cutting out on full throttle in 3rd and 4th momentarily until I lift off. Otherwise it is fine and appears to have no problem in lower gears.
I have changed the waste gate solanoid for another used one and cleaned out the (oily) dump valve. Cleaning the dump valve made it smoother and changing the waste gate made the cut out less violent but it has remained.

Any ideas ? remap? another solanoid ? anything ?.It's really spoiling what is otherwise a top drive