I’ve searched on here but can’t find anything similar to what is going on with my 08 F150. I just had a rebuilt engine put in back in February but it was doing this prior to that. I changed out the fuel filter but no change but I did notice that they used the coil packs from the old engine. After about 20 mins of driving the engine surges, the rpms will go up about 300-400 with less than a second between surges, it will do it in all gears and reverse but once i stop and it cools down just a little it’s fine. Today it did it after a lot of running around town but actually stopped and ran fine.

 

I have a 2010 5.4. I got gentle surging when using "flex" fuel. It went away when I went back to regular gas.