Problem with high temps. growing in grow tent in garage please help

Sorry Ive been posting a lot lately, but im a newbie and am learning. First a little info. for you guys. Im growing in a 8x4x6.2 grow tent which is in my garage. Im using a 1000w mh light. I have installed a duct system with a passive intake and fan for venting the exhaust. The duct system is obviously ran though the light to help cooling. I have also installed a portable ac unit which I think is about 9k btu. (plent big enough) My problem is my garage is well insulated and theres no vents (except in the attic) The intake/exhaust system is pulling air from the garage and exhausting it out into the garage. (which I think might be one of the problems) and the ac unit is inside the tent and venting into the garage. The temperature is fine in the tent as long as I run the ac all the time which is going to cost me way too much $$. As soon as I turn the ac off the temp. goes up 10 degrees or more in a matter of an hour which I know is not good for the plants. It does have a timer, but like I said as soon as it shuts off the temp. goes way up. Ive got a door into my back yard which I leave cracked open but it only seems to help a little at night and im nervious about leaving it open in the day. Any suggestions please!!! One thing I thought of is extending the intake hose for my exhaust system to draw in cooler air from outside instead of the hot garage air, but I would have to leave my garage door open a crack which is fine if im home, but I leave for work at 630 and get home around the same time. My plants actually seem to be pretty green and healthy at the time. (at least I think they are about a foot tall at 5-6 weeks from seed) I can supply pictures if needed.