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