Q: Miss Fire Cylinder 4

I know I have the infamous Vulcan 3.0 V6 Ford Engine with possible bad exhaust valves. However let me explain my symptoms and why I think its something else( or at least i hope so). I am getting a check engine light for P0304 and sometimes P0316 in combination. Which is Miss fire on Cylinder 4 and miss fire on startup first 1000 revolutions. When the car starts it starts and idles absolutely perfect and smooth. As i drive it and get it up to operating temp it starts to shake slightly at about 700 RPM's while idling. If i ever so slightly reeve the engine to about 950 RPM I can feel the engine shake a little less. If i go to 1500 RPM it runs perfect with zero shaking(so smooth you cant tell the engine is running). It drives and shifts perfect and does not have a loss of power. The longer the Engine runs and get warmer the more it shakes at idle. newplugs and wires within last 100 miles. Engine does not tick, etc I suspect possibly a plugged fuel injector? or fuel system issue.
My car has an automatic transmission.