Kitchenaid Speed 2 too fast, can't adjust it

"Take a look at the speed control board and see if anything looks burned, The are also adjustment screws on the board:

KitchenAid KSM150PSER Speed Control Plate Replacement"

I know you are converting 220v to 110v, but what is the FREQUENCY of the electricity in your German city? 50hz or 60hz? I know from personal experience that it is different, and eventually gave up on converting electricity for anything with a motor (especially clocks)