1991F150guy

Hi, could anybody possibly tell me any reasons why a 91 f150 with the 5.8, 351 would idle very unevenly? If it's in park on a flat surface it will idle around 1500 RPM's and jump up and down. But as soon as I put it in gear it drops back to a normal idle speed around 800 or 900 RPM's. I can't seem to think of any reason why the motor would do this so any help would be much appreciated! Thanks in advance!

78fumminsman

Fuel Pressure Regulator came to my mind, or even the Fuel Pump possibly?