My truck will pulse or surge.. whatever you wanna call it but it does it when I put the truck in park or neutral. But its not all the time

 

I had a similar problem in my 1998 F-150. It turned out to have multiple vacuum leaks on the 4.2 liter V6 engine. I replaced a PCV elbow near the throttle body. My mechanic replaced some PCV lines on the back of the engine. That didn't solve the problem, so they replaced some gaskets on the engine. Don't quote me, but I believe it was the ladder gasket (upper to lower intake gasket), and they may have replaced the gaskets between the lower intake and the cylinder heads. They said something about replacing the seals on the bolts that hold the upper intake as well.
My truck was idling rough and surging. It produced codes PO171 and PO174, which indicated a lean mixture on both sides of the engine. The problems started small but got worse over time until I got it fixed.