What year Mustang are you talking about? (Hint: create a SHORT signature like mine) If it's older and has thermal gauges, then that's pretty normal for Fords.
Typically the voltage regulator on the back of the instrument panel goes bad. I've never seen later magnetic gauges do that, but I suppose it's possible if the sending unit goes nuts.
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch