Power Light blinking but can wake up?

I have a Dell Dimension PC running Win2000 for over 2 years without
problem until recently. I've noted that after a period of idle, either
login or logoff, the power light sometimes will blink/flash and the
monitor light turns orange (no signal). As I understood, this means
that the system is in hibernate (standby/sleep) mode. However, there
is no respond when I move the mouse, or hit any key on the keyboard
....just like it's hang. The only way to regain control is to reboot.
Noted that I do not enable hibernate or standby under my account and
in the system (checked regedit). There is no change (both hardware and
software) as before, except the internet connection changed from T1 to
Cable. Any idea what may be the cause and how can I fix it?