Volvo - 240 :: 1988 - Losing Power / Rattling In The Engine

I have a 1988 Volvo 240 DL station wagon that has been in my family since just before I was born. It has somewhere around 300K miles on it (the odometer broke 7 years ago at 250K). It drives around town more or less fine, usually. But sometimes it loses power on me. Usually this happens when I take it on highways, up big hills, in situations where I need to accelerate quickly, etc. It just starts to slow down, and I have to downshift and ease slowly through whatever stretch of road I'm on until I can get back up to speed (usually on a downhill). It's not totally consistent--sometimes it has this problem, other times it doesn't.

The car will go through stretches where it runs fine, then I'll have this problem on a highway, and then it's a little more likely to happen for a period after that (even on surface streets). This has been happening in some form for a few years. When I first had the problem, it happened continually until I heard a terrible rattling in the engine. When I took it in, they found that a spring in the piston/cylinder had broken off and shot up, breaking the cap off one of the cylinders (sorry for the lack of a more technical explanation).

When they fixed that, the problem seemed to go away for a while, but eventually came back (though there has been no rattling since). The most recent time, it had been running around town just fine. Then I took it out on the highway and really lost power--had to slow it down to 35-40mph going up a hill on a 55mph highway. For the rest of the drive it struggled up hills, even as low as 2nd gear. But on my drive home (about 1.5 hours later), the car ran smoothly (though I continued to baby it, and there were fewer upward hills). Since then, even around town it sometimes feels like it might lose power again, but it never becomes a huge issue because I haven't demanded a whole lot of the car.

After that long-winded explanation, I'm wondering: could this be something as simple as spark plugs? Or is this definitely a bigger issue with the engine? If it's a cheap fix, I'm happy to go for it, but if it will require putting several hundred (or more) into the engine, I'm inclined just to let it limp along until it's finally time to take the car out back and shoot it. Figuratively, of course.