There has been numerous reports on random NULL reference bugs with v3, especially on unstable shared hosts or on servers with many umbraco sites. We finally found the bug which is related to cache references and are currently testing on a larger number of sites. We'll commit the changes Tuesday this week to the source repository on CodePlex and release a patch version 3.0.4 this week (unfortunately we can't easily add the source to the repo right now due to 3.1 dev being the main branch on cp. We'll add the full source code as a separate download to the release).
Sorry for the inconvenience, but the patch is an easy, non-destructive, assemblies only update which should only take a few minutes and won't affect your site in any way (other than obviously get rid of the NULL reference bugs).