New Grinder Started Running Slow.

I have owned this new Metabo grinder for about two weeks. I bought it still in box from a local millwright working a job that does not allow personal grinders on the job, so I got a great deal on a NIB Metabo. Happy days for a while until, while in use, the grinder started to slow noticeably. I thought it may be overheated so I let it cool completely before trying again. It started at full speed but slowed again after a couple seconds of contact. I pulled the front to see if there was any excess play in the pinion gear and noticed that the clear shim(clear plastic) had only 3 of the 4 screws through the shim. It turns out there were two of the shims and the factory had missed one corner on both and the loose film had been sucked under the ring gear and was bound up under it. after freeing the shims from under the gear, I chose the better looking one and replaced the front carefully inserting the screws through the shim and hoping that I had discovered the problem. No such luck, any suggestions on what to try next. Love the tool, it's my first Metabo and I hope it's an easy fix. It doesn't sound mechanical (no grinding noises), could id be electrical? Would it be warranty covered even if I was not original purchase? It was still New in Box.