Mac OS X Lion Server screen freezes frequently

I have recently purchased the Mac Mini Server with 8GB RAM Upgrade and have been experiencing screen freezes frequently. The Mac Mini is connected to a ViewSonic LED Monitor via an HDMI cable and is running Mac OS X Lion Server.

It all started a month ago and I suspected it was due to an external hard disk I had. When I removed it the problem didn't crop up for a few weeks. So I thought the issue was resolved.

Over the past week the problem re-surfaced again and nearly everyday I am experiencing screen freezing about five times a day and the only option is to power down my Mac Mini Server from the power button. This is becoming increasingly frustrating. When screen freezing occurs, everything freezes and I get some small rectangular boxes on screen moving.

The only things that have changed over the past week was that I installed the latest Safari fix and connected the Harman Kardon Sounsticks II to the Mac Mini.

Can't understand what is happening. From various threads I read, many mac users are experiencing screen freezes. If anyone has any resolutions please let me know.