I take it you have no code. Could be the throttlebody sticking. My son had the same problem on his 2006 5.4. It finally threw a code and it was indeed the throttle body hanging up and not reacting to the pedal action. It has been an issue on some 5.4's especially in cold climates which I don't believe you reside in but it may not be a true factor.
Last edited by DYNOTECH; 02-28-2011 at 10:21 PM.