You could be having a problem with the gauge that was not uncommon when E36s were new, it's a bit odd that it's happening with a car with some years on it. There was some sort of electrical problem, and the solution for me was given by the dealer's service shop, pull fuse #30 or #11 (I'm not sure on those numbers) and leave out for 10 minutes. Reinsert. Cross fingers and see if that solves it.
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