SMU reset fails to fix Network problems

I shut down my iMac G5 for 2-3 weeks (but left the mains power connected). On rebooting, both the Ethernet & Firewire connections were missing ("the Ethernet/Firewire cable is not plugged in"), "built-in Firewire" was unchecked and greyed out in the Network Port Configurations pane, and the Mac could not be put to sleep from mouse or keyboard or start button.

I did an SMU reset as per knowledge base article HT 1767 and this fixed the problem - temporarily. It keeps coming back, and sometimes needs two or three resets ("Ethernet has a self-assigned IP address and may not be able to connect to the Internet") before normal function resumes.

There is a second, possibly related, problem. After restoring normal operation, issuing a Sleep command may randomly put the Mac into "semi-sleep": the display goes blank, the hard drive keeps running (Ethernet light on ADSL modem still green), the case gets very hot (CPU stuck in a loop?) and the fans do not operate. There is no response to keyboard or mouse or Start button, the only escape from semi-sleep is to cut the mains power and do yet another reset.

I ran the Apple Hardware test - it finds no faults when operation is normal, but stalls after 2 minutes when the Ethernet & Firewire connections are missing.

Also, booting in Safe Mode or Open Firmware or resetting the P-RAM achieves nothing. Date and Time is correct and Airport always functions.

Can anyone suggest a permanent work-around?