Fuel cutoff- what triggers it?

[Orin Eman]

> 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.

Torn wastegate diaphragm?  Does the top of the wastegate
hold vacuum?

Orin.