Two months ago I changed out the plugs for the first time on this car, a 2000 C280 with 60m. The car had developed a slightly rough idle and I found that 2 of the plugs off of the same coil were fouled. After new plugs, the car ran fine but now it has that same slight roughness at idle again. I haven't pulled the plugs to see if the same ones are fouled again as I suspect but does this sound like either worn spark plug wires or a failing coil problem? Or something more serious like a compression problem in these cylinders?

Thanks,

glenmore
2000 C280
1991 300CE
1990 LS400