Valve cover gaskets won't keep oil from going into the spark plugs wells; that's controlled by the spark plug tube seals. So, unless you replaced the valve covers, themselves, you can't completely rule out the possibility of oil leaking into the well unless you pull the coil packs out. Another possibility is a fuel injector leaking down, which could cause a hard start and foul a plug, which would burn off as you drive it. A fuel pressure leakdown test would confirm that. A bad crank position sensor would also affect startability and could creat misfire issues. So, it may not be just one problem and you may have to do some diagnostic work to figure out what problem, or problems, exist. Cleaning the MAF is a good start. Make sure your battery connections are clean and tight. Check for any potential air leaks, like splits in the rubber intake duct or split/disconnected vacuum hoses. Check the fuel pressure. Keep in mind you have more than one cam sensor. Also, a ignition coil pack could be failing and may not be triggering a code, which could create the symptoms you have.