2003 Hyundai Tiburon Supercharged P0304 Code: Injector or Fuel System Issue?

I'm posting this in the right section as I believe it's related to fuel injection and engine performance. I own a 2003 supercharged Hyundai Tiburon 2.7L, recently upgraded with larger 360cc injectors and increased boost pressure via pulley modification. I'm now running approximately 9.5 psi, which is well above the required level, and fuel trims remain stable. After a dyno tune, the car began throwing an intermittent P0306 code—typically appearing when driving below 5 mph. I replaced plug wires and spark plugs, checked all coil packs, and swapped out the original injector (custom-bored, bench-tested and cleaned). After installing a new 360cc injector, the P0306 became more frequent, and the vehicle entered limp mode. I then moved the suspect injector to cylinder #4—immediately triggering a P0304 code. After replacing the injector on cylinder #4 with another new 360cc unit, the issue persists intermittently but is now less severe than before. This strongly suggests the injector is at fault, yet I have no evidence of internal blockage or failure in the new units. The symptoms follow the physical location of the injector, which makes mechanical issues like valve or cylinder problems seem unlikely. All other injectors are functioning properly. I’ve considered a lean condition between open and closed loops, especially since I run slightly rich fuel mixtures for safety and use methanol injection. However, fuel trims remain balanced. I’m currently 8 whp shy of reaching 300+ whp on the dyno and would appreciate any insights into diagnosing this P0304 issue. Thanks in advance for your help!