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Hey John,
This is a common problem with many homeowners with rooms above the garage. There really isn't a solution to completely remedy your problem. Here are a few suggestions to help alleviate the harshness of the extreme temperature changes.
1. Check the insulation above the room and make sure its at least R-60.
2. Insulate attic kneewalls if you have some with R-19 and then with foam board.
3. Air seal any holes from wires, pipes, vents or ducts to that room.
4. Insulated Garage doors help a lot.
These are some things you can do yourself that will help make your room more comfortable. It's a good idea to hire a professional to perform a whole house energy audit and complete HVAC system check.
We have had a lot of positive results with these options and I hope they help you too.
All the best,
Charlie
Aladdin Insulation and Home Improvements