1985 380SE with strange idling/surging...

I've had this car for about two years now, and it's only got 125K miles on it. I've done a pretty good job maintaining it, but now it's doing something that I just can't figure out.

When I first start the car and drive around initially, everything is fine, but after the car heats up and I've driven it for a while, it begins to feel like one of the cylinders is not firing. The engine begins to "rock" every once in a while (once every 3-5 seconds). The exhaust seems to come out in staccato puffs, though it's not smoky and does not smell or anything like that. If I try to accelerate up a hill while this is going on, through the gas pedal I can feel the engine hesitating for a second and then running fine for a second. Sometimes it gets so bad that you feel like you're riding around on a bumpy road or like you're driving in a manual with a kid that just can't get it in gear.

I pulled off the air filter the other day and cleaned the throttle body and linkages with carb cleaner. I pulled a couple of spark plugs and they seemed to look okay -- no oil, just slightly red, not a lot of carbon buildup, etc.

After having driven the car around for a while, the car exhibits this behavior regardless of whether I am stopped, accelerating, decelerating, or in gear or out of gear. I have tried various gas octanes, and have run a few fuel system additives in recent tanks.

The maintenance cycle seems to indicate that the fuel filter should be changed, which I have not done yet. It is also probably due for a spark plug change.

Anyone out there have any idea what might be the culprit? I really love this car, but I'm a college student without enough money or time to baby it like I want to.