sometimes fast , sometimes slow
 

when i start the car after being parked for 2 houres my x (2001 4.4 ) is powerful and alive. after continuosly driving it feel slow . i tried to use steptronic mode and reset the ''throttle memory'' but the x is also weak.
im not getting any error code (no service engine soon) but the computer is showing a poor mileage (12 mpg)
could it be a bad maf sensor or cam sensor? sparkplugs?
nb: the x does not stall