i have a 2001 ford 7.3 f250 with 500,000 miles on it (i know its crazy!) and after i was idling it for a while i drove off and the truck just lost all its low rpm power. i would hammer the throttle and the motor would growl up the rpms slowly till it reached 3,000 rpm then it pulled like it should. after i turned off the truck i let it sit for 45 minutes started it and it drove like nothing ever happened. what is wrong?