Hey guys, I was in a similar situation as well. Have Stillen Gen 3’s on my Z and while I was tuned on Uprev I started getting P0300 codes randomly, coincidentally after getting a new set of OEM Denso spark plugs. I ended up reaching out to Eugene Turkov in Concord, CA who solely tunes on Ecutek. He reviewed my data logs and was able to eliminate the issue by removing the Uprev tune and retuning me on Ecutek, the DTC is still active if there’s ever an issue so the code itself is not disabled. I haven’t run into the issue since making the switch to Ecutek.
If any of you are still having trouble with the code popping, I’d reach out to him. He’s an Ecutek master tuner so you’d be in great hands, apart from the fact that he also works for NASA. He supports remote tuning as well so don’t worry if you aren’t anywhere near San Francisco, CA. Here’s his website:
http://www.infiniticare.com/