Hopefully it's just a spark plug. I had that problem... my first step was to replace the spark plugs when I realized that my valve cover gasket was no longer rubber but hard plastic that fell apart in my hands. I replaced that, hoping that it was the problem (my plugs were in great shape). Nope... it was a blown head gasket. I ended up getting a new (ok, lightly used) engine (wasn't really worth fixing a +250k mile engine). Have you noticed anything weird going on with your coolant or oil levels? If not, it's more likely that it's just what witeshark said.