Don't laugh, because I know that there are all sorts of problems it could be with a 1999 F-250 SD, 5.4 V-8 with 261,000 mile on the original motor, but here are the symptoms and my corrective actions:

Symptoms:
1. P0171/P0174 & P0403 Codes (especially after refueling ??)
2. Random misfire (previiously occurring - before DTCs)
3. Rough idle (occurred with codes) smoothed out after PCV replacement
4. LOUSY acceleration (most noticeable after replacing PCV/cleaning MAF)
5. After replacing PCV and cleaning MAF - low idle when cold (at or <500
rpm.

Corrections:
1. Found that freakin' elbow from PCV behind manifold...with a hole big
enough to put my finger in, replaced elbow and PCV valve (old valve
seemed to be fine, but I figured I'd replace it anyway (mistake?)
2. Cleaned MAF sensor on K&N intake and another sensor (resistor?)
downstream of the MAF.

After replacing the PCV, I reset the codes, and don't have any new ones yet..with emphasis on the yet. Could a bad EGR valve cause low idle, or IAC/IAT, or even a vacuum leak? I don't believe that any of the vacuum lines have ever been replaced, and although I maintain it well (look at the mileage!), I neglected replacing things that weren't broken. Any ideas would be appreciated.