Another Newbie here, I have been having a problem for a while with my Arduino Duemilanove Freezing.
Please don't feel offended, but you provide almost no useful information, except the usual "it doesn't work". Unfortunately 'we' get to read that quite a lot here.
To make more than educated guesses, we need more information.
What arduino software version do yo use, what operating system, exact error messages (not quoted from memory), ... etc. etc. Please be as detailed as possible.
Please elaborate on the "30 minutes" issue. Did you test that time repeatedly, what about e.g. 5 minutes? Are you sure it is a timed issue and not just the unplugging/reinserting?