Fluctuating Temperature gauge

[mboucher70 at hotmail.com]

Any ideas on this?  Start the car from cold, coolant temperature rises steadily over a 10-15 minute period.  Just as its approaching normal temperature, it reverses direction rapidly and goes from needle almost straight up to fully cold in about 30 seconds.  Warm air continues to blow out the heater core so I know its not actually cold.  Fan hasn't tripped on yet, so thats not an issue.

Then, a few minutes later, temperature gauge restarts its rise from fully cold, and is back to warmed-up (needle straight up).  From this point on, everything functions normally.  I could drive the rest of the day and its not going to repeat the pattern.  If I get stuck in traffic, the fan eventually kicks in and cycles normally.

Next day, if I start the car from cold, it does the same thing.

I'd suspect an electrical fault or grounding problem if this was repeating during a long drive.  But since it only happens once, on the first warm-up cycle of the day, I'm thinking it must be something else.

Thanks for any ideas

MC
Audi 100 1990, 2.3L I-5 NF engine