2000 GMC C2500 idle control

Code p0507 "rpm's too high" tested the idle circuit for resistance and was well within the guidlines. (55 ohms) sometimes the idle will drop to 500 rpms while in gear but in park. Sometimes 700 and sometimes 900-1100. No other codes come up so is my pcm going bad. Also. Used to start right up but lately. It cranks a little before firing up. I'm so frusterated that i'm ready to put a match to the truck