My GN once had a problem with ignition at EXACTLY 5 pounds of boost. Didn't matter how I rolled into, nothing mattered. It would backfire and have ignition problems at EXACTLY 5PSI, no matter what.
Turns out, it was fuel injectors. The end cap glue joint failed inside my fuel filter. All the rust in my tank from sitting got dumped into them at once. They were so bad, Chuck couldn't test them first because it would have fouled his injector machine.
It was amazing because it felt EXACTLY like ignition.