Thank you for your response Zango.
We can't upgrade it to Debian 5 because we also run Plesk on it, will get too complicated for just a VPS. The memory is fine, constant at around 500 MB with about 1500 MB free.
We've been thinking about using a cron job, not yet integrated it, but it doesn't matter that much. It's just too bad when we have like 32 people in one of our servers (so the server is full) and it goes down, it'll take a few hours before it's full again and then a couple of hours later the server goes down again.
We run with version 1854, I think we need to update it We will try later today.