I have a 2000 f150 v8 4.6 and while driving the rpms go up and oil light comes on. The mph gauge drops. When I press gas pedal it does nothing at all. But motor just races. Then mph goes up and continues to drive fine.

 

Sounds like the transmission is slipping. Check the transmission fluid while the truck is warmed up and in park on FLAT ground. Make sure it's full and it should be bright red NOT a brownish color. If it's brown it needs to be changed and possible repairs or rebuild required. Good luck.