I have a 1991 f150. My truck surges in park at operating temp. I have replaced vacuum lines, egr valve, egr position sensor, throttle position sensor, idle air control valve, distributor cap, rotor bug, spark plugs and wires, and intake manifold gaskets. what else would cause it to at idle in park?

 

First try disconnecting the battery and let it sit unpowered for 10 or 15 minutes. This will clear the computer. Reinstall the battery terminals and start it up. If it's still doing it, you might have a timing belt that needs to be replaced. Also, see if you can pull any codes from the truck with an ODB reader. If you don't have one, most auto parts stores will read them at no cost.