heat hand--very strange

I was out tonight driving around and on my way home I noticed that my heat hand would drop all the way down and then back up to normal position. It did it quite a few times but it never once went up like it was over heating. This is the first time I have ever noticed it doing this and there is no check engine light either. If you need to know it's a '91 hb with a d15 block and y8 head. Any Ideas? Thanks

Thermostat could be getting stuck open.

Could also be your temp gauge sensor or loose wire. Is there any corresponding fluctuation in the heat output available from the heater when the gauge is moving? In that case I'd def go with TSC and check the thermostat. GL