There is a noise coming from one head, like a bent pushrod or a damaged lifter... maybe a weak spring.
I was thinking at a relationship between the mV difference and some mechanical issue because the problem starts only after a certain RPM.
OK I know: I should check a lot of other things. This engine has now 100'000 miles, been supercharged during 30'000 and I can't expect everything is still perfact.

What would you do with this engine? Just let it work as long nothing bad happens or consider a rebuild? Or buy a low milage rebuilt engine? I've never rebuilt an engine and I should buy a bunch of tools. Chances are I spend a lot of money and mess up the engine