I find the same thing on mine.....04/4.4. it takes a few seconds to respond when cold. mind you this is only after it has been sitting for a few days.

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2004 4.4i Titanium Silver
Eisenmann Race Exhaust
Sirius/Nav/BT/Aux/DSP
4.8 Body Kit (Rear only so far)
ACS Pedals
Aluminum Running Boards
DINAN High FLow CAI
20" Style 87's