2006 LS2 out of Chevy Trailblazer SS in a rock buggy running E40 ECM. Running stock tune without VATS or emissions. Has been running this way for years now. Runs great, exactly as it should, only EXACTLY every third ignition cycle. The other two ignition cycles it runs like crap, apparently in some sort of reduced power mode, but there are no codes or check engine lights. When it's running right, it will continue running right until you shut it off. To get it to run right again, you simply flip the ignition switch on and off twice and then on again. If you try to run it on the first or second cycle, you get reduced power. We've come to just flip the ignition switch twice every time before starting it out of muscle memory. It's 100% reliable in the pattern. The only way the pattern is broken is if you disconnect the battery, which starts the pattern over. If you shut it down and then flip the ignition on and off once and then try to start it again months later, it will still be waiting on the 2nd cycle before it will give you the normal power on the third.
Weird, right? Any ideas?