Mine acted identical to what you describe as my cracked head was getting worse. The level would go down under higher rpms, then return close to normal when idling. It drove me crazy since the light would be on when on the highway, then go off when I slowed down. Finally had wifey bring the rpms up while I watched the coolant tank and sure enough you could see it slowly dropping. I chalked it up to air (combustion gases, actually) getting trapped in the system from the crack in the head.

Hope yours is something as simple as a bad coolant level sensor (definitely a possibility), but I would be checking for cold cooling system pressure. If you come out in the morning and your top radiator hose is stiff, start budgeting for a new head. Sorry to be the bearer of potentially bad news...

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Keep everything as simple as possible-but no simpler--Albert Einstein