So a few days ago we started getting a bunch of 500 errors on our website. After calling 1&1 they told us to change it so we were using php5 by forcing it in our .htaccess file. We did that and still had 500 errors. After a few more calls and helping them out a bit, it was determined that some of our php scripts were pushing us up against our threshold for a shared server with 1&1. After disabling some plug ins on our main site, we finally tracked it down to a site we switched from Drupal to WordPress. It seems since we created our WordPress theme from scratch we had some bad php coding in there causing us flood our php usage and therefore getting 500 errors intermit. It was not a constant error but something that would show up a few times an hour per user. Anyway to help you out, its most often your php scripting causing the error or going over your CPU max threshold usage as well. When I asked about a VPS from them, it seems you have to move all of your databases over yourself, not something I really want to do. Since without server access it would be real hard to track down our php error, we are just going to start from scratch and do a new theme.