I suppose it could have gotten dry. The thing is that when I got on my bike right at the start all was well, quarter mile down the road and I was at 225. In reading the manual tonight I think I may have one possibilty I hadn't thought of. Electric cattle fences. The ride was in a small town (Union, IL) and there were quite a few farms. I remember seeing some cattle, and smelling them! When I was reading the manual the light went off when I saw the comment about electric fences.
I went for a run tonight and everything was cool with the HRM. Also I am ruling out other riders as there was no one around me when I started out and things went haywire.