240 losing power

A couple weeks ago, during a day trip on the hottest day in August, my 1990 Volvo 240 DL came close to overheating -- which isn't unusual for this car. I got it to cool off, but then it did something it's never done: it began losing power/speed. If I released the gas pedal and then depressed it, the car would gain a little more power, but not enough to keep it going. I pulled over and turned off the car, waited 20 minutes, and made it home without any more symptoms.

At the same time, the rattling that happened before I last had my catalytic converter replaced is back again (though, when I had it replaced I also had rattling exhaust parts welded), and there's a burning smell coming from under the hood, though I think that's head gasket leak dripping onto something.

Since the near-overheating-loss-of-power incident, the car has sputtered/lost power just a few times. Is this a fuel problem? Is it the catalytic converter?

Sounds like your catalytic converter may be clogged. It's a common thing, especially if you're losing antifreeze through the head gasket. Antifreeze in the exhaust would cause the CAT material inside to collapse thus causing the exhaust to clog. The resulting buildup of pressure creates all kinds of problems. It would cause the overheating and the lack of power.