Air-con making engine self rev??? 1998 B5 Passat

I have noticed that in my passat the air-con is seemingly connected to the engine. If I press the 'AUTO' button quick enough I can rev beyond 4000! And now it has started to make the engine under-rev itself e.g. down to 500rpm. I was at the mechanic a few weeks ago with the same problem and he says he fixed it by changing the air-fuel regulator (I think :) ) Now it is back :(

Does anyone have any ideas about what is wrong? Should I sell and move onto my next VW?

Thanks, Peter