Low boost/lack of power MC2-engine (MC dual knock)

[Dag Bøsterud]

I have a late 1990 Audi 100 TQ with the MC2-engine (also known as the MC dual knock-engine).
I have replaced all sensors including both knock sensor, the idle/full load-switch and the air filter.
If I try to read fault codes, I get 4444 = no fault codes stored. I'm quite sure I don't
have any turboleaks or any major vacuumleaks.

The car is in very good condition. The mileage is 131 000 miles. The car is original, never had any
modifications done to it in any way.

The problem is that sometimes the turbo doesn't seem to boost at all, or at least according to the
gauge in the board computer. This occurs - if it occurs - after driving for some time (30-60 minutes).

Normally the gauge shows 1,4 at full boost, but when this occurs it shows 1,0, and I can feel
that the car feels more sluggish. Not that it is a monster when it boosts 1,4 (0,4 bars), but when it
supposedly doesn't boost at all? I'm sure it boosts something, but it's not a lot..

What could cause this?

/Dag
Norway