This is a 1998 c70 hardtop with the 2.3L turbo engine ~225k miles.

I gave the engine a pre-emptive tune up back in March. I bought the full Volvo kit from FCP including new air filter, spark plugs, wires, cap, and rotor. I gave it an oil change and put in a magnetic oil plug for good measure.

Gradually since March, I've started getting CELs. Initially it was a misfire on #2, which would later clear up. I checked the sparks and wires and boots to be sure nothing was loose, but it never solved the issue permanently. The problem had gradually gotten worse, with CELs showing up much more frequently with misfires on #2, then #3, and now #5, plus the random misfire code.

I pulled my cap and checked it for trouble. Rotor was very secure, nothing funny inside the cap. A bare trace of oil around the edge, but nothing obviously wrong.

What's really odd is that the engine is running just fine. I get no detectable feeling of misfire at all, yet I can't seem to avoid the CEL for more than a day or two. When they do clear themselves out, I can see them pending on my scanner tool. Seems to be inescapable, and I've no idea what to do - aside from dropping another $160 on a new tune up kit and hope that I manage fix it.

Any ideas? This is starting to drive me a bit crazy.