500 error messages keep coming!

I have had a problem of 500 error messages with my site http://www.wereyoutherewhen.com. My server, 1&1 first said that the Video Sidebar Widget was causing the problem, which was a shame as I had completed all the pages using this. So I took them all off and embedded them in text bars. I completed a third of the pages with this when suddenly I started getting them again. Now they say that it is the Custom Sidebar, which I had actually deactivated anyway. They say if it isn't that then the site must just be too heavy for my package, obviously meaning that I should pay more! Can you suggest any solutions, please