1997 7.4L 2500, 230,000 rebuilt engine. used run like a top... never been abused or pushed hard.

Here is the problem
My 1997 starting to run rough, slugish at low RPM, as if it is missing,
-Tuned it up (plugs, wires rotor and cap)-----------No improvment
-engine ligh came on, code p0174. so ...
-cleaned Mass Air Sensor, new air filter, cleared code -----------No improvment
-Checked Fuel preusser regulater, according to Hayen (sucky book), it is not performing correct, so i changed(it was a bitch)---No improvment

No sure wut else to do, can anybody help.

thanks
Slm