2003 Taurus 3.0l duratec with P1506 P0300

Thanks to all in advance. You were right on the money the last time I posted here. :)

Started looking at my step sons 2003 ford taurus with the 3.0L duratec v6. The car has 68Kmi and stumbles under load or climbing a hill.

several mechanics have looked at the taurus and with $600 worth of repairs the same issues occur. The mechanic also says the next step was to replace the catilytic converter. So, I have taken it upon myself to try to fix the car.

by the looks of the documentation, they appeared to see p0300 code and replaced the following:

1) plugs and wires with motorcraft
2) coil pack
3) body assembly aaf 3f1z-9e926-aa
4) air filter
5) body gasket

The car was used when he purchased it. Looking at the regulator, it and the rubber tubing appears to be brand new. The rubber tubing between the line side and the regular appears to be older which is why I make this observation. This may be why the got rid of the car in the first place.

The MIL is on and when I initially pulled the obd II codes via a laptop, I found only a p1506 code
P1506 Idle Air Control Over Speed Error.

I cleared the code and we took a test drive. P1506 did not return. I noticed the fuel pressure per obd II was 38psi, acc on, idling, and driving. We then replaced the fuel filter. It was horribly dirty gas that poured out of the line side. The drop side appeared to yield clean gas. Replacement of the filter yielded a 2psi improvement and now show 40psi. Per the repair manual this is still low and it says 50-56 psi running and 45-56 key on engine off. I can't seem to locate the fuel pressure access point on the injector rail or anywhere close by to verify obd II sensor readings.

we performed another test drive and event that he said normally occured happened. The stumbling occured, the MIL came on, and remained on. I pulled the codes and found a p0300
p0300 random misfire

Sitting in the yard I brought the rpm to 3000 and the car began missing. At 4000 the missing was horrible and the car will not go above 4000 rpms. I also noted the fuel pressure dropped to about 28psi when the stumbling began. It almost immediately recovered the fuel pressure to appx 40psi, but the stumbling continued.

if the car has been sitting a while, grey smoke is observed on start up but quits after 5-10 seconds or so. Not sure if this has anything to do with it, but thought I would mention it anyway.