Once my engine has been run a while and warms up the idle begins to go up and down. You can watch the oil pressure gauge go up and down as this happens. It only does it once the engine has warmed up. When I first start the engine its fine. Its just after its been run a while that this starts. Any idea what could cause this?

 

Check your throttle positioning sensor if bad it will cause your engine to "hunt for idle".

 

ITs either hunting for idle.
Or something isn't fireing properly which causes it to jump up and down like its going to stall.

 

could be running rich, check tail pipe for black sutty condition. also might check idle control valve. check codes. haynes book shows you how to test most of your sensors. check out my thred (running rich) dated 11/17/2003 it might give you a few ideas of things to look at. mine is still running rich, but im working it..

 

I would start with checking codes then just for the heck of it clean the Idle air control valve. I do mine every 30,000 miles. Take it off and remove the solonoid and spray it good inside with foamy window cleaner and run your finger or a qtip around inside it then rinse with hot water. You may have to do it twice but this works great. Make sure the oring and gasket are good or replace them if not.
If that fails to help then check for vaccume leaks. Doing it when it's hot sounds sortof like a plenum to intake or in the lower intake maybe but I doubt that is it. It's just not that common on a ford. Chevy yes ford not often. Well not on the older ones. How many miles on it? has it had a proper tune up lately? by proper I mean by the ford book or your owners manual service schedule. It says to replace or inspect a lot of things that never gets done till it's too late. Mine has gone over 600,000 miles following the extreme service guide in the owners manual and never left me stranded.

 

Just an update:

Now it seems to idle fine but the oil pressure guage keeps going up and down when idling. Its stays put when I'm driving.

 

Oil gauge could fluctuate because of bad engine ground or the screen on the pump sometimes carbons up. B12 Chemtool in oil right before oil change helps but breaking it up this way sends particles around the engine and can stop up lifter holes etc. History of your truck can help determine if the gauge is malfuncting or oil pump is starving. Or you can hook up a mechanical pressure gauge for a true reading and see if it moves same way. I'd also suspect TPS for erratic idle. Other than that spray cleaner on the throttlebody with air hose removed and idling to help smoothe idle.