My friend owns an American car too and he is of the opinion that the AC Delco service center (Salwa road or Wakra) is the best option for American makes. There are other good garages too such as American Specialist Garage in Barwa village, Building 18. My personal suggestion would be Tristar Garage, Street 24, Gate 242, Tel: 44505574, Mob: 33755840. They are very qualified and a well known garage in Doha. I haven't been there yet but now that my car possibly needs transmission work, it's the most reliable and promising place that I have found after lots of research. Best of luck.