What makes Audi A6 2.4 1999 to shake before it reaches 80km/h

When it's in neutral and you hit the gas, does the rpm rise slowly? And if you try to give it too much throttle does it choke and try to die? It sounds like your catalytic converter is partially plugged, or there is some other exhaust restriction. If the check engine light is on, there may be relevant diagnostic codes available, some of the auto parts chains used to read the codes for you for free, but I'm not certain that is still the case.

AC compressor put load on engine causing engine shake vibrate.but check make sure AC compressor mounting bolts tight and drive belt correct tension.now if AC compressor very old most time bad compressor cause more load on engine causing shaking real bad.

Could be spark plugs or a bad ignition coil (but the dealership should have checked the coils). I would replace the spark plugs and air filter (just to be safe) and see if the problem goes away. Both are pretty inexpensive and easy to do. Presumabely your dealership checked for fault codes in the engine and didn't find any?

regars for all ideas but i think need to check the electric of all car and make sure all the leds &wires are connected well.
hope it help

need to have diagnosed can be anything from rough running engine to front end problem to transmission with information given

you have 1 or 2 bad ignition coils very common on this engine

Get your tyres balanced, makes a HUGE difference and the tyres last longer too.

you have the cam and crank sensors to check. also driving like this can ruin the cat so that should be checked as well. and the o2's. the cam and crank sensors can be checked on a scanner or scope or check there resistance if you have what the resistance values should be. there are other things that can cause this a fuel filter clogged up to many things but check the cam , crank sensors and items like fuel filter ect plus cat and o2's they should be swithcing when repair is done above and below 500 rich to lean lean to rich. make sure they are not stuck.

HAVE THE TIRES BALANCED AND CHECKED FOR SEPARATION

A very common problem is faulty tires or incorrect wheel balance of the front tires. Check for a bulge or a wear on the tires. If all is in fine find a mechanic to check your wheel bearings. Check for a broken strut assembly or shock absorber. Check for tire rod bushings and the balance rod. Check complete front suspension.