Help! forced restart...

I have and iMac G4 with the following:
Hardware Overview:

Machine Name: iMac
Machine Model: PowerMac4,2
CPU Type: PowerPC G4 (2.1)
Number Of CPUs: 1
CPU Speed: 800 MHz
L2 Cache (per CPU): 256 KB
Memory: 1 GB
Bus Speed: 100 MHz
Boot ROM Version: 4.3.5f1
Serial Number: QT2130UWLF4

For whatever reason I keep getting these error pages that tell me I have to restart my computer (in several languages). It doesn't appear to be software specific and happens only sporadically. I'll be working, surfing, importing and the screen will stop, go gray and a prompt will tell me I have to restart and how to do it.
What's going on?
I recently bought this computer used from a friend. He wiped out the harddrive and we added my admin name and downloaded iLife. We ran disk warrior at the time (about 4 days ago) and everything came up clean.
Any info or suggestions would be appreciated.