:driving: Turbo Boost Gauge readings

[NoDOHC]

I hate to say it again, but this sounds like a vacuum leak (it may be a mal-adjusted bypass screw in the AFM - if S5 Turbos have it).

If the engine revs when the TPS is pressed, it must be getting air from somewhere. It is probably running really lean and the Idle adjust screw is turned out so far to keep it running that it really is idling with that poor of vacuum.

Are you pushing the part-range or full range TPS? (I think that the part-range TPS is pressed when the throttle is shut, so I am guessing that you are pressing the full range TPS).

My theory is that you are pushing the full-range TPS in, which is causing throttle enrichment, which is allowing your woefully under-fueled engine to rev up. This also explains your idle vacuum issue (a large vacuum leak has no air going through the AFM when there is significant vacuum because it all comes in the leak).

Places to check:
Input to BAC Valve
Injector Pintle cap seals
emissions removal plates (if you have them)
cruise control (if you have it)
Vacuum Booster for brakes.

Let me see if I can describe what the engine is doing.

If you try to crank it without pushing the gas pedal, it will not start.
If you rapidly pump the gas pedal while cranking, it will try occaisionally (almost always as the gas is being pressed down) and will eventually start.
Once running, it revs to about 2,000 rpm, at which point it is misfiring so bad that it dies.
On the way down, the engine catches about 800 rpm, it fires several times in a row which causes it to rev to 2,000 rpm again.

If this is what it is doing (which I think you have already described) then it is running too lean to fire as it revs up. This is almost a dead giveaway for a vacuum leak. The only other possibility is that the FPR is referenced to a ported vacuum source (which shouldn't make this drastic of a change).

Edit: Things to try.
Press the Full Range TPS in and hold it in with the BAC unplugged. If my theory is right, it will rev for a little while, reach 3,000 to 4,000 rpm and start to stutter and eventually return to the loping idle.
Take off every vacuum line except the one to the MAP sensor and the FPR and plug the holes.
Make sure that you have the correct AFM for the car.
Get more carb cleaner or a smoke machine
Watch your fuel pressure (I doubt this is it).