It's not heat, your engine is running rich and the spark plugs are fouled. Clean out the PVC valve, if you find a plastic screen in there throw it away.
Then look at all of the small vacuum hoses and check for cracks. A small leak will really mess up your engine timing.

You could also have a dirty throttle body, probably has never been cleaned. Let the local Jackson mechanic know about the potential leaks and let him clean out the throttle body. All he should need to do is clean the spark plugs, unless you haven't changed them for 40K miles.

He could also plug up a computer and check the efficiency of the O2 sensors, if they are looking bad it is because of an air leak.

Klaus
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Why are Volvos so endearing? Its just a car.