When I'm sitting at a light, my idle will be about 500 RPM, when I put it in park it will jump to 600. Occasionally I will be sitting and it will surge and the rpm jumps to 700, and will stay there for quite a while. Any ideas on what may be causing this?

 

Clean the throttle body real good and that fixes it most of the time. Might be a TPS "throttle postion sensor" . Get a can of throttle body cleaner, remove the air intake and give it some good spray's Have someone idle it up from inside and keep hitting it. Shut it off, floor it and spray some more.

Or remove it and clean on the workbench.

 

What size engine do you have?
FF

 

I have the 4.7.