2002 Ford F-150 Poor Engine Idling

Have 02 Ford F150 w/ 4.2L V6 and approx. 106K miles. Truck will not start/idle unless gas pedal is pressed. MIL is on and read a DTC of P1504. Replaced IAC valve with no change in performance. Sprayed some carb cleaner into the throtte body and truck finally idles although low and it still needed help starting. After the truck warms up, it idles low; around 500-650RPM. When under load (AC/Lights on) the engine loads down and idles real rough and surges. I've checked and rechecked the DTC and it always comes up with the same code. I don't want to do hit or miss troubleshooting. The original O2 sensors are installed and as best as I can remember had EGR valve replaced when the truck was under warranty. Have ran Seafoam through the vacuum system and added some to the gas. Until last week, truck ran great with the MIL coming on ocassionally but faults always cleared themselves. Prior to problem, truck was sitting for about 3 weeks. Any help would be greatly appreciated.