1997 5.4 loss of rpms
 

I have a 1997 F-250 it will drive fine for an hour or so then will lose rpms I can push the throttle to the floor and no response what so ever will only idle. I have a bank 1 too lean code I checked all the vacuum lines and they are fine. I can turn it off and start again will drive ok for 10 to 30 mins but gets worse as I go. It only does during the day so far and not at night. I have no clue what is wrong and neither does to dealer! PLEASE HELP!!!!