Dropped a new long block 4.3 Vortex into my 2000 Chevy Blazer (2WD). Starts up great and runs great at idle. When I take the RPM to around 1500 or higher I notice surging and it will throw a P0304 code. I searched the forums and am still stumped.

--I've replaced cap, rotor, and plugs(all AC Delco).
--Had the timing and Crank variation done.
--All the main sensors tested within specs (Oxygen, Map, Maf, TPS, Air temp sensor).
--Cat converter is brand new.
--I checked the EGR and the EVAP purge valve and they seem to be seating and working.
--Compression test showed good with 150-160 on all cylinders.
--I also replaced the fuel injection system with a new updated MFI spider. Did a fuel system test and is testing great (holds 61 with KOEO and around 56-59 with engine running).
--I hooked a vacuum gauge up and at idle was running 19 lbs with no drift or fluctuations.
--Had the coil and ignition module checked at Autozone and tested good(had them run alot of tests to heat it up )
--Saw on the forums to try a spray bottle on wires/plugs when dark and saw no light shows lol

At idle it doesn't set any codes and runs smooth. But If I take the RPM's up to 1500 or above then I'll get a noticeable surge and it will throw a P0304. When I unplug the MAF sensor I still get the surging but not the misfire code.
Could the system still have a vacuum leak and not be seen by the vacuum gauge (if I somehow put the intake gaskets on wrong)? Could the Cam Retard/Crank Relearn be wrong even if it doesn't set codes?
Any help/tests/comments would be appreciated.

 

Try a different spark plug in cylinder # 4 Maybe the factory made a bad one? Or the gap got knocked in? Make sure the wire is snapped on well on both ends?

 

Put another new plug in there and changed around the wires from another cylinder. Same thing

 

Put in a new fuel filter, air filter, and cleaned MAF. Still does it.
Is there any way to check the timing to make sure it was done right?

 

You can check what the indicated timing is with an appropriate scan tool. On the newer computer controlled distributors, it is not as easy to check timing manually as it was on the earlier models and since timing is not adjustable aside from pulling the distributor and advancing/retarding by a tooth or more...