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.

Several months ago, the rattling that happened before I last had my catalytic converter replaced came back (though, when I had it replaced I also had rattling exhaust parts welded). It's most noticeable when accelerating at low speeds, or driving around a parking lot. There's also a whirring/ticking sound coming from underneath the car, and a burning smell coming from under the hood, though I think that's from an oil leak.

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? Do I have several unrelated problems?