Karl,
If this is only happening after the motor warms up and is under load, it would sound as if you might have an electrical problem along with the above mentiond possibilities. Was it doing it before and is it just under load or will it do it while you are stopped if you rev the engine up? Also does it seem to be out both sides or more on one side? Maybe pulling a couple of plugs and looking at them.
Ernie is an expert now at blown head gaskets and sticking valves. He sould be able to tell you how to test for that.
Ron