My Audi is 'bucking/kicking forwards. it does it periodically, always in the 3rd gear range when it does it (eg: while passing through third gear, and while cruising in 3rd gear)

it does not seem to have anything to do with the temperature of the engine, as it does it periodically, about every 5 minutes, or whenever I pass through 3rd gear, from when I first start driving the car to when The engine is heated up. I originally thought it seemed to do it more when the engine was heated up more, but after observing it more, it does not appear to have much correlation, except that the bucks appear stronger after the car has been driven for a long time and is very hot already.

I have taken it to my mechanic, and they did a transmission fluid flush, and replaced the radiator, and upon picking up the car, it bucked again within 5 minutes of driving it. The mechanic seems to be confused as well, and now they say they cannot diagnose it at all without the check engine light coming back on.

any ideas on what this may be?

and where to start out diagnosing it myself?

some ideas that were thrown out by the mechanic were; ignition coil, or valve body replacement. however I do not have much more confidence in this mechanic, so any new opinions would be much appreciated! thank you!

Christina