I have an 06 Cummins and every once in a while it will start idiling weird, like tonight when I got home I put my truck in park and was waiting for it to cool down for a sec and it was idiling under 500 rpms. The main problem is when I put the truck in park and the motor will idle fast the slow. But it only does it once in a while. Need some help on what it might be

 

Any codes in memory?

My WAG is an injector or harness issue.