03 Tiburon P0304

Not sure if this is in the right section. I've got a an 03 supercharged Hyundai Tiburon 2.7. Recently upped the injectors and psi with pulley. I'm running about 9.5 psi on 360cc injectors (more than adequate), and fuel trims are good. Right after the dyno-tune, started throwing an occasional P0306, usually when going less than 5mph. After I swapped plug wires and plugs, checked coil packs, replaced the injector (these are custom bored injectors, bench tested and cleaned). After installing a new 360cc, the code became more frequent, and car kept going into limp mode. I moved the speculative injector to the #4 cylinder, bingo, threw a P0304 code. Obtained another injector, and replace the #4 cylinder one, and it still throws it intermittently, but it's not near as often as the previous injector.

It seems too obvious the injector's at fault, but I know the source, and they're tested. I doubt it's a valve or cylinder problem as the code follows the injector location. The other 5 injectors have been fine. We had discussed maybe there's a lean situation between open/closed loops. I'm running slightly on the rich side for safety, also running methanol injection. I really need to straighten this out and hit the dyno again. I'm about 8whp shy of 300+whp.

Any help is appreciated.

maybe a blockage in the rail? although if moving the injector moves the code then it kinda leaves little other possibilities!