2.0 JTS problem - sooty exhaust/little power

[sjn2004]

Mentioned this before but still not got around to taking the car in. 156 SW 2.0JTS on a 05 plate.

- After 3500 rpm engine seems to go flat on power or even less power.

- Noticed today that the exhaust is very sooty

- Trip computer is showing my average MPG as 27.4 over last 2000miles.

Somebody mentioned a knackered oxygen sensor before, so after 3500 not enough oxygen is being let in resulting in unburnt hydrocarbon deposit in exhaust system. Does this add up?

Opinions/comments welcome.

[dr chuff]

Possibly Lambda sensor, or knackered air flow meter.
A good garage should spot either without too much trouble.

Dr C