While im driving. The RPM gauge will drop for about 15 min and return. Also when I shut the truck off. Somtimes it will not start back up, just turn over. Again I have to wait 15 min or so and then it will start

 

Sounds like crank pos. sensor, but scan for codes or do key trick, cam sensor has same symptom

 

2000 models have a cam and a crank sensor. my 2000 reg. cab did the same thing and it turned out to be crank sensor. the ecm on 2000 models uses the crank sensor as the primary sensor for timing and the cam for back-up.