Theme vanishes seemingly at random

This is a weird one, I think. There are times I’ll hop onto my blog, and find that the theme I worked so hard to modify is gone and has been replaced by the WordPress default (Kubrick). When I log into admin, the theme is still there, it’s just not active. It’s easy enough to fix by just re-selecting the theme. When it first happened (I believe I was using WP 2.0 at the time) I thought it was the version of the software, so I upgraded (needed to anyhow). That didn’t fix it, so I changed themes thinking something in the code on my previous theme wasn’t written properly. Today, it happened again just like with my old theme. For a while I thought it happened sometimes when I didn’t properly log out of admin, but I’ve found that’s not the case either.

Here’s a link to my site: Startup BizCast

Any ideas? I’d hate to think potential customers are finding my site looking like that.