I had a similar issues with the volume button on my inspire (same as dsire hd). I took it apart to check the volume flex cable and the upper part of the cable had a slight rip. I went to this link and ordered a new one, popped it in and it worked fine.
http://www.repairsuniverse.com/htc-desire-hd-main-flex-ribbon-cable-replacement.html
yours sounds like it could also be an issue with the physical button as well. I would open it up and take a look.