Removing the intake tomorrow.

Going to inspect pressure regulator, do a compression test and just generally look for antyhing suspicious. Seems to me that at least one cylinder should have low compression if a head gasket is the cause.

Have a code reader that gives live engine readings also, running that on the drive to the garage I have for a few days.
I anticipate at least a day delay on any partds I need once the intake is off.
May remove lower intake if there appears to be any reason to go deeper.

Failing to find anything, may just drive it till the problem gets worse. The cause would have to be more apparent if the symtoms get worse.

Still baffled at absolutely no error codes.