Theme deactivated at random intervals

My theme is being deactivated at random intervals – what I mean by that is I will check in on my blog every 12-24 hours, and all of a sudden its contents will just be empty.

I will log into wp-admin, and discover that under “Presentation” no theme is selected! I then just click my theme, and the blog works fine again. I come back 12-24 hours later, and the theme has been deactivated once again. Any ideas?