I recently purchased a C200 Merc with 162 000 Km. On two ocassions the car lost power and began to idle roughly. It would not accelerate at all and barely moved. A few moments later everthing went back to normal and seemed ok. It was a hot day and I was using the air conditioner. The previous owner reported having this problem as well. Any advice as to what it could be?

A backfire usually indicates a timing issue or having the wrong plug wire on a spark plug.

SOMEBODY HAS A VACUUM LEAK PROBLEM (emissions) Have the ECM scanned & take it from there.

It sounds like some thing you have done if the fault started after the tune , did you check the gap on the new plugs, check the firing order as it may be messed up . 4 cylinder 1342 6 cylinder 153624 in direction of rotation.

Air filter, ECU, O2 sensor, bad EGR valve, or clogged cat converter.

Onboard diagnostic computer should be throwing codes. Autozone can check these free (as will other part stores).

check map senser if thats good check your fuel filter

check the spark plug gaps. [I think it should be 0.8mm ] too small or too large - for surging [ incomplete combustion ] . rough belts- squeel ? check correct tention & spray some penetrating fluid like CRC on the pulleys . But for the **** I really dont know what to suggest

Coils, wires, plugs, fuel filter. A real tune up in other words.

the most common causes are plug wire, plug, vacuum leak at intake, fuel injector failure and last no/low cylinder compressio.

Sounds like a possible vaccume hose leak?

Try this. Turn the ignition to position 2 and leave it about two minutes(you should hear components moving/making noise under the hood). This should relearn the throttle stop(sounds like your vehicle lost synchronization). After that period of time, start the car and see if it helps. If not, then it will probably have to go to the dealer so they can use the computer to try and relearn it.