I got a voltmeter from a pick-and-pull which did the same thing. I found a better voltmeter which didn't have a sticky needle. It is probably a worn or dirty pivot on the pointer shaft. It doesn't look like there is a way to open up the meter and clean it.

I noticed mine came up to about 10.5-11 volts one morning (in the red I think) when I put the key in. I thought for sure my battery was dead, but the car cranked fine and immediately went up to the 14V range when running. Every since then it's been fine and shows around 12V (well above the red) with the engine off, so I don't know what happened.
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Volvo 240 Links

Robert:

Mine does the same thing, particularly when the temp goes below about 30 F. I suspect a deteriorating electronic part of some kind in the gauge itself, but I haven't been concerned enough about it to check yet. The car is an '88, but the gauge is from an '84, so it's the same as yours.

Either way, I don't think it's an actual voltage problem. If the voltage were really running over 16, as the gauge says, we'd both probably have some fried electronic stuff right now.