Hello, I just picked up a 99 F250 SuperDuty 5.4L with 140K on it. It drives pretty smooth, but has a hesitation under certain loads, particularly in OD on the highway up hills, and when I step on it. It kind of clears itself up though, back and forth. No Check Engine lights. I changed the plugs (OEM motorcraft), fuel filter, air filter, pcv valve, and oil changed. It still acts up. Any pointers?