I've been getting some strange behavior from the temp gauge on my '90 535i. A couple of times now, while stopped at a light, I have noticed that the temp gauge has dropped from its normal position at the halfway mark almost all the way down to the 1/4 mark. As soon as I take off from the light, or even with just a good blip of the throttle, the needle rises right back up to its normal position, and stays there. I can't manage to force it to do this. It's just happened seemingly randomly a couple times in the past week.

Any ideas?