I have bell hi-spd.(up to 7 mbps) with a speedstream 5200 modem. Recently my computer has begun shutting down without warning... similar to a hard crash but with much greater speed. It sometimes takes several attempts before it will power up again and then it's fine for a few days but then the cycle starts all over again. Has anyone else experienced this and does it sound like a modem or a computer issue (Apple G5... 4 years old)?
Any help is greatly appreciated.

Not modem.

sounds like the power supply