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I have this problem for several years now with my MK4 with AEX engine and now I decided to solve it.
The problem is that with the AC on the idle RPM arent stable. When the compressor turns on the RPM drops from the normal ~900 to about ~300 and the engine almost dies. Than the RPM playes up&down arround 1k and drops once again..and over....and over again.
When the AC is off - no problem.
When I am driving (not idle) I dont see any problem (thou it might exist but harder to spot).
I tried cleaning the throtle body and reseting the ECU. didnt work.
Any ideas?

To me, it sounds like your battery is on its last legs (I know its been years) but the alternator is doing a good job....get your battery tested, I'm sure it's free at places...or near free EDIT * Just noticed you're in Israel, so I can't really say if its free

I can imagine A/C to be a big drain on a battery.

Thats coming from a none expert opinion, definately an electrical problem though.

no, its isnt the battery. The battery is new (changed it a couple of month ago) and same the still problem. I think that it could be the alternator....I think that I'll disconnect the alternator and let it run on the battery for a few miles and see what happenes...