Mac keeps falling asleep!

Hi guys, hope someone can help me as were currently having a nightmare at work that is preventing us from doing any work!

I came into work today to find our server asleep. Our server is on an old G4 tower. We usually keep this running 24/7 to allow for remote access, but for the last 2 weeks we have set up a scheduled shutdown over the weekend to try to keep the server fresh and clean - bad idea!

Every 18 seconds or so, the machine goes to sleep. The monitor goes black (with a green flashing light on) and the tower goes dead silent and displays a glowing power button. Moving the mouse or pressing a key restores everything where we left off, but 18 seconds later, puff, its gone again.

We managed to get in and turn of the scheduled shutdown but this hasn't resolved the issue.

Whenever the screen is on we can access the server (not email) but as soon as it goes to sleep, we get kicked off the server.

Any help would be amazing because we're currently unable to do any server based work (majority of what we do!).

Thanks in advanced.

Lee