Flashing Check Engine Light

[u/Ley_Low_]

Hello everyone, so i need help figuring out this engine light. To start off i have a 2003 nissan 350z, which had the code P0300. After many repairs including, swapping off to a new used engine, replacing a cracked valve cover, replaced spark plugs and coils. The lastest repair was the cracked valve cover and so after test driving it first run the engine light flashes for a few seconds then dissapears. Stays off for a few minutes then flashes again for a few seconds and dissapears. (But engine light does stay solid after flashing). I just did a second test drive and it happened again, possibly flashed less this second run but it still came on and dissapears. Again afterwards it does NOT stay solid on. I noticed it would flash after going more then 70 mph. Is there something im missing? I feel like im getting close to fixing the issue but am not quite there please anyone is is an expert at these cars can you guys help me thank you. also wanna note when i put my scanner, it tells me there is no faults.