Throttle body fluttering
 

2007 F150 5.4 with 265K idle will fade down to 300RPM and also hesitate when accelerating. Thought it was a vacuum leak. sprayed starting fluid everywhere, no change. Can hear fluttering in the area of the throttle body during the idle fade.

Did you spray along the air cleaner assembly? I mean from the Throttle body to the air cleaner. If there is a crack or split in that piping it can cause issues with how the engine runs. My airbox retainer opened allowing the air filter to fall partially out and my truck ran like crap, might be worth a look

Don't spray starting fluid, use an unlit propane torch.