I have a very weird power thing going on:

If I hold the throttle down half or 3/4 way or so and I'm not climbing a hill (engine not under stress) and I keep it at a constant rev in the same gear for several seconds (5-10 seconds) I will suddenly get a 20% boost in power. It feels just like I've stepped down on the throttle the rest of the way, or that the fuel was being restricted but then suddenly released. If I give it full throttle, it won't make the power jump and will continue to climb in revs but be very be under-powered or plateau quickly and keep me barely at highway speed.

Even more weird: If I let off the throttle at any time after the power boost has occurred, it goes back to the way it was and I have to do this technique again to gain back my 20% lost power.

To be clear though, this boost is actually power that's being lost otherwise as it feels back to normal during this boost.

I have a turbo pressure gauge and can watch my needle at, say, 5lbs, suddenly push up to 9 or 10 as I gain power. This leads me to believe it is not a turbo issue and definitely fuel related.

I've done the usual stuff already. New fuel filters (twice!), new fuel lines after the lift pump. ALDA is backed all the way out. Turbo feels perfect, hoses are tight and good. No air filter issue that I can see. Valves are perfectly adjusted. Im noticing any smoke issues. No different sounds.

The problem is consistent and repeatable. If I am in my final gear it wont do it usually as it's struggling to reach 60mph and the engine is under stress. If I'm on level ground or going downhill slightly, I can get it to do it in the final gear which suddenly makes keeping up with highway traffic much easier.