Q: My iMac (Snow Leopard) randomly freezes.

Only a laserprinter is connected. Sometimes  I can work for a few weeks without problem, but then the computer  freezes. The computer sounds 3 monotone chimes in a row and repeats this, keyboard an mouse become inactive till I do a hard shutdown.

Took the computer 2 times to the Apple store, they tested for a week but apparently nothing happened then.

Anybody any suggestion?