2000 Honda Accord V6 P1399 Code: Diagnosis & Missing at High Speeds Below 2000 RPM

I own a 2000 Honda Accord V6 EX Coupe with 168,000 miles. The car ran smoothly for two months (approximately 2,000 miles) before developing a recurring issue. Recently, I've noticed a noticeable 'front and back' vibration or missing sensation when driving above 50 mph and below 2000 RPM. This behavior only occurs under specific conditions—no symptoms at idle or lower speeds. The throttle feels responsive when I lightly press the pedal, but RPMs remain steady around 1800 RPM. Only when I push harder does the RPM jump to about 2200 and the jerking stop. The car doesn't miss at idle or during normal driving conditions. I recently scanned the vehicle and received the following diagnostic codes: P1399, P0300 (random misfire), and P0301 through P0306 (individual cylinder misfires). I'm confident that all six cylinders aren't experiencing full misfires—my spark plugs show a light brown to reddish color, indicating proper fuel-air mixture. I've already replaced the spark plugs and performed a drop test on each ignition coil. The results showed strong pulse readings at idle, confirming that the coils are functioning properly. I also tested cylinder 6 with a plug-out method—no change in RPMs, and the coil produced excellent sparks, indicating no issue with ignition. I've ruled out valve tapping (no noise), fuel system problems (plugs burning correctly), transmission issues, and EGR valve malfunctions. I unplugged the EGR vacuum line, but that did not resolve the problem. The car previously ran poorly after using Walmart gas for two months; switching to Shell gasoline, adding Lucas Octane Booster and Iso-Heat cleaner improved performance significantly. The EGR port was cleaned at 110,000 miles, and valve adjustments were performed earlier. I'm now considering whether a fuel injector failure—particularly on cylinder 6—is the root cause. While it may not fully misfire, it appears to intermittently stop delivering fuel, which could explain the vibration and P1399 code. I’m hesitant to spend $72 on an EGR valve replacement unless absolutely necessary. A Honda dealer charges between $80–$108 for diagnosis, which feels excessive given my findings so far. I’d like to get this resolved without costly repairs. Any insights or suggestions would be greatly appreciated—especially regarding fuel injector testing and PCM-related issues.