Fuel cutoff- what triggers it?

[TM]

Hi listers,
I've encountered a rather interesting problem.

The car in question is an '86 5000CS quattro, MC engine.

My car won't rev past 4400 rpm, at which point the fuel
cuts off and the car jerks rather violently.

The car feels _much_ more powerful recently, all the way
up to 4400 rpm.

Boost gauge in the trip computer won't go past 1.2 bar,
although it would do 1.4 bar previously. Throttle body
hose is in good condition and less than a year old.

Any ideas?

Thanks,
Taka