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

Before you go, if you have the time and desire today, I'd suggest you pull off your entire air intake and see if there's any clog. Clean it out and see if it responds. My guess is that one of your air hoses has become clogged. It doesn't take to long to figure it out.

To pull off your cold air intake, you just push the hose towards your car until it disconnects from the plastic housing on the front grill. Then pull it out. Then disconnect the vacuum lines from the box as well as the large hose that goes to the engine. Disconnect the MAF and undo the top. If your filters dirty, or there's tons of crap there that might explain it.. Otherwise you can take the box off by pulling up and towards the engine (its more or less set in there snugly..)

Check that as well as your hot intake for buildup, You never know where it all comes from...

If everythings peachy you might want to remove the big air hose to your engine as well as your throttle body, cleaning it won't hurt.

BTW I don't know how advanced a DIY'er you are but this shouldn't take much more than an hour and less than half the second time you do it...