Sorry, this has just jogged my memory - my B5 did exactly this. Once I'd figured that it wasn't an actual problem, I just ignored it, and did have any other related problems, so I don't think it's the body computer. Also the B5.5 sport that my dad had *started* to do this sporadically in the month or so before he sold it.
I asked the chap I sold the B5 to, and a year down the line the temp gauge is still operating intermittently, with no other problems having manifested, so I wouldn't worry too much.
I think it may be a genuine TADTS