I have a 1999 Ford all stock F150 4.2 v6 with 165k miles that idles perfect but under light load misses and shakes but under heavier throttle does not miss. No codes, replaced with motorcraft plugs and wires, air and fuel filter, coil pack, tps, cleaned maf, replaced dpfe, pcv, replaced decomposed pcv hose. I used to be a master tech for Ford but I was not a drivability tech. I ohm the new motorcraft wires and they ranged from 9.2k to 11.8k ohms. Trans works perfect and fluid clean I do not suspect shudder. My next suspect is FPR or possible lean condition on one or more injectors. and goes away when computer commands more injector opening time under heavier throttle. Any thoughts what else to look at? Update, Installed new fpr and new ground for engine cleaned IAC, EGR, I did notice misfire gets worse as engine gets warmer. Still no codes

 

Have you tried injector cleaner in the gas? Techron is the best. Have you checked fuel pressure with a gauge? I assume you used a Motorcraft PCV valve?

 

check the ground on the coil pack, if you reused the original 7mm bolts they may be rusty and not grounding the coil pack to the valve cover thoroughly...it's a longshot though was once a similar problem for me with that symptom

 

I have seafoam and techron in tank and also seafoamed intake. Pcv is motorcraft. Cleaned egr,throttle body, removed throttle body and checked for carbon. found intake pretty clean. I may need to run a new ground strap and see if that might help.