Well, start by draining the oil to the correct level. Take the plugs out and clean them up as well. If that doesn't fix the problem the only thing I can think of is a new head gasket. What type of plugs did you get when you replaced them? I don't think new wires or plugs is the problem since you replaced them last year.

Oh, and NEVER go to Walmart to have anything automotive related done. They're on the same level as Jiffy Lube.