Re: Garage is hot as an oven all summer!
It is interesting that the garage is open to the house attic. This should be a code required firewall. If it were me, I'd install a firewall to close off the house from the attic. This will not only prevent fire from rapidly spreading from garage to house, it will also slow down heat transfer and other issues. Frame the partition on 24" centers, insulate it, and install at least one layer of 5/8 drywall, preferably two layers for added burn through protection. Joints and fasteners just need one coat of joint compound and tape.
As for venting the garage, do you have skirt vents around the perimeter? Again, this should be a code required thing. If so, then you can install a power vent either through the roof or the gable end, which has a thermostatic switch to operate it, I would personally include a master override switch so you can turn it off if you need to. The thing is, though, you've got to have the perimeter venting in order to have supply air for the power vent, otherwise you'll create big problems with back drafting appliances housed in the garage.
Other options would be to leave the roll-up door up 3" and the side door to the yard open to allow air flow and fume exhausting.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!