Ibiza TDI: AC Issues & Performance Problems

I'm having a small problem with my Ibiza GT TDI, manufactured in 1998 (110 hp). Whenever I turn on the air conditioning, it's happening more and more often (especially when it's very hot outside) that the car jerks when accelerating and only has about half of its power. When idling, the engine vibrates briefly, and the RPM drops by about 200, but then immediately increases again! My friend thinks it might be the viscous coupling of the air conditioning compressor, but we're not exactly experts!
Thank you for your help!!
Monchi.

The air conditioning system has a magnetic coupling. If this coupling is broken, the air conditioning system won't even turn on.

I suspect your compressor is stuck (did you forget to add oil when you changed the refrigerant?).