Mac Mini Freezing

In the last few days my Late 2009 Mac Mini with 10.6.8 Snow Leopard installed has begun to freeze after a few hours of no activity. I have set the computer to never go to sleep as I thought that was maybe the problem. But it did freeze again. It freezes when I leave the computer unattended for an hour or so but has not frozen while I am using it. I just reset the SMC (System Management Controller) by shutting down and then unplugging the computer for 30 seconds or so. I am hoping maybe that is the problem. Anyone have any ideas on why this might be happening and how to fix the problem I would greatly appreciate!

Thanks!

Jeff