Audi 80 problem

Hi,

I own a 10 year old Audi 80 SE.
For the last year I have been experiencing a small problem with the car.
I drive my car everyday on the motorway doing about 30 miles a day.
Occasionally the car will start to judder and lose power, and when this
happens I can only get a max speed of about 40-50 mph, no matter how much I
press the gas pedal. It will stay for a about two or three minutes and then
the power will just snap back suddenly. It has never stalled.
No warning lights come on, so I have no idea what could be causing this.
I think, but I'm not sure, that the car seems to behave better when the tank
is full of petrol rather than half full.
This only happens about once a fortnight, and I don't think any one activity
makes it happen.
Can you please help with some suggestions, thanks.

Cheers

O.


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