I've been having the same problems with increased frequency. It's pretty ridiculous - I've been running wordpress on dreamhost for years, same set up, very little content, only about 20-50 unique visitors a day and I'm getting 500 errors all the time just while trying to make new posts or edit tags, etc. Even deleting a spare theme in the backend! Or just visiting my stats page.
Have any of you moved? Any good alternatives? I'm starting to feel they are just not the solution.