My only suggestion would be a bigger grinder. If you're having trouble with Metabo grinders, there may be no help available.
I've had nothing but good out of the old 5" Makita and a 6" Metabo grinder, if you have any use for a bigger grinder. We've gone to using 3M grinding pads with the plastic backing disc. The quality of a grinder will become apparent pretty quick with those. Can't grind a groove with one, but they will remove a mountain of metal in a hurry.
Now, if you're looking for a good electric pencil-type die grinder, Pferd is where it's at. Metabo is as bad as the rest with that particular tool.