Stumbling and hesitation under boost

Ok, while this car is not an SVO, the same principles should apply. I have a 1987 Thunderbird Turbocoupe. The problem I am having is that when full boost kicks in, the car starts to stumble, it surges a little bit, and just doesn't feel like it is giving full power. If the engine can get beyond about 4000 rpm, it will seem to pick up a little, but its inconsistent. I can feel a very definite surging, as the engine struggles to overcome the issue.

A basic history of what has already been done. The car has new spark plugs, wires, the intake has been cleaned, the gaskets have been replaced, the PCV seems to be functioning, no leaks in the intake are to be found. The timing belts has been replaced, and the timing has been adjusted. The car spools up quickly, and in first gear the problem does not seem to be present, not sure why, maybe not enough load. Every gear higher than that, the problem is very apparent, with the issue becoming more obvious in higher gears. The car is a manual 5 speed to those who are wondering.

I don't want to blow lots of money fixing things that aren't broken, and down want to spend tons of time chasing ghosts. Some other things worth reporting, the exhaust manifold is cracked, not sure how much effect that would have.

Ok, replaced TPS, had no effect at all, but there was enough evidence to convince me the TPS was bad, it dis not have a smooth progression in voltage, it had erratic spots that told me it was due. But it has not resolved the roughness and stumbling from about 3K-4K.

I do have a Gillis Boost Valve, is it possible that it bleeds enough to throw off the computer? Or could this be a problem with the turbo?

Ok, I found the problem, the spark plugs did not have the right gap for the boost I was running.

I had it at .035 and I changed it to .030.