I have a 1999 Audi A6 quattro, have just changed original fuel

On the side of the transmission.

1999 Audi A6 Quattro 2.8 V6 engine

ALL 1999 Audi A6:
A6 Quattro Sedan
A6 Quattro Wagon
A6 Sedan
have the 2.8L DOHC V6

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UNDER BACK SEAT

This is a manual transmission? If so the clutch probably needs to be replaced. When you say "VIOLENTLY" what is going on, what is the road speed, what are the RPMs at? The more specific information the better. And this problem ONLY occurs between gear changes, right? The other times, everything is normal?

Sounds like the gas presser valve or weak fuel pump. When it dose it again turn the switch on and leave it on for about 3 mins. and see if it will start. If it dose I would say the pump is weak.

did you plug in the fuel pump at the fuel pump. did you cycle the key off and on at least 5 times check the plug at the fuel pump first to see if its still plugged in

You would have to go to the dealership unfortunetly.

Do you also see the shuddering when you hit the gas to pass someone or accelerate quickly? If so, I'd swap out the fuel filter first - less money and that's a symptom of a clogged fuel filter.

If that doesn't do it, then yeah I'd check the mass air sensor out.

http://wiki.ross-tech.com/index.php/16795/P0411/001041
Check this site for any Audi codes. This is a problem with the air that it injected into the exhaust system to help burn any uncombusted fuel. Check the air pump located in front of the pass front wheel on the bottom of the engine. It is pretty easy to find. The air pump has a suction hose that goes to the air filter housing so that it has filtered (clean) air, follow the hose