On the way home tonight, luckily close to the homestead, the '99 V70 started running really rough, like a cylinder wasn't firing. Sure enough, after about 1 minute, the check engine light started blinking, then went solid. Barely got home, and the error code reads P0304, cylinder 4 misfire.

So I pull the coil pack looking for an obvious culprit, and there doesn't appear to be any. Plugs are pretty new, but coil packs may be original. I cleared the code, and switch cylinder 4 coil pack with cylinder 2 coil pack to see if the code would return for #2, but so far, I can't get the computer to post another code again! The car runs exactly the same with the coils switched, but I cannot tell if cylinder 4 is still misfiring (bad spark plug?), or if cylinder 2 is misfiring (bad coil pack?) until the code posts.

What else can I look for that may have caused this code? Is there way to force the computer to post the code, or do I have to let it run continuously until the code comes back?