Mine does the same thing. Someone here had a lengthy post detailing his attempts to fix it. His details mirrored mine- Old house, old non-grounded electrical system, his gust recordings were in multiples of 14 (14-28-42-56 etc). After much troubleshooting, it came down to him grounding the computer itself. So I think DanS is on the right track.
I am finally going to ground my system directly over the Memorial Day weekend (too much snow at the cabin this year!), I am confident that this will eliminate the weird gust info.