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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Fuel filter.
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Conor

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The fuel tank is ventilated via a carbon filter. Have a look there or open the filling cap for a sec after 1/4 tank is used.
Ronald

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Cheers, where do I find the carbon filter, and why is this not changed as part of the Audi service? O.

much
used.
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