Timing! Why didn't I think of that. In this day of distributorless ignition, you forget about timing. It'll be the first thing I'll look at if I can find my timing light.
Someone mentioned possible clogged exhaust ports to the EGR valve. The 4.9 doesn't have pass-thru exhaust passage thru the manifold (intake) like other cars; it just has one exhaust inlet for the EGR. The passage from the EGR valve to the floor ports under the throttle body are clear. My question is, what's at the head-intake junction gasket? Is it a square hole the shape of the port, or a small hole that's easily clogged?