AHU Loss of Power.

Hi People, I'm new to the forum and hope someone can help me. I have an A4 with a AHU engine, it has 169,000 miles on it and I have had no problems for the last 20,000. I am experiencing loss of power, if I drive around town and keep the revs under 3000 the car is fine.If I get on a dual carriageway or motorway and accelerate hard past 3000rpm after the second or third time the car goes into what I think is limp mode,it won't accelerate past 3000rpm. I drive through all gears but can't get past 60 on the speedo. No warning or diagnostic lights illuminate and I have had the fault memory read, no faults are recorded.if you turn off the ignition and turn straight back on the power returns, until I accelerate hard again. This fault is getting worse every time I go out. I have checked vacuum pipes, no issues found, I have replaced the fuel filter and air filter in case of blockage but still no fix. No manifold leaks or visible problems, the fact the power returns after the ignition is off/on leads me to think a sensor is at fault or receiving an incorrect reading, putting the vehicle into limp mode. Any help would be appreciated, anyone got any ideas, are there any common faults.

cheers Andy.

p.s off to work so won't be able to reply until tomorrow.