Fan accelerates, then goes into sleep mode?

Help!!!

This is an add to what I written below. I carefully opened the computer and turned it on. It has been running for over a 1/2 hour and it does not go into sleep mode. Fans that are running are the: (1) over processor, a Verax system; (1) System optical fan; (1) System fan on end of Hard Drives is not on at all;

if there is a fan in the power supply it does not appear to be generating much air flow.

My Computer:

Power Mac G4 867 MHz DP with 2 GB Ram, OSX 10.5.8. It has (2) internal ATA bays for Hard Drives, and (2) for Ultra ATA Hard Drives. (2) USB and (2) firewire ports on Computer.

(2) USB on Cinema 23" display, (2) USB on keyboard.

Problem:

This all happened within a matter of weeks.

Started having problems when i plugged in a lot of devices, including: on keyboard, camera and mouse USB; on Display, USB Keyboard and USB headphone jack; on Computer - firewire (2) LaCie 500GB HD; and on Computer - USB printer and scanner (never run at same time), and sometimes another USB HD.

First USB's did not always work, such as headphones. Then firewire might not be recognized, then computer with firewire plugged in started to go into sleep mode. Now fan runs very fast and loud, and then goes into sleep mode: if shut down for awhile takes a while; if using a lot maybe within minutes.

So then I replaced one HD that had been non-functional for long time,with 120 GB new, and made it the master, and things worked fine.

Then ran with firewire, started having problems again.

So replaced old 60GB HD with new 500 GB, from Memory X, they confirmed compatability, was constantly going into sleep mode.

Left computer off for 2 days and it has been on for about a half hour, internal fans ran really fast

and loud, and now has gone into sleep mode. If woken up wants to go back into sleep mode. If I leave it for a while can keep it out of sleep mode longer.

Questions:

Is this a power supply problem or a motherboard replacement

or something else?

If the motherboard, can I upgrade from 867MHz to 1.25 GHz?

Is there a way for me to test this? Link?

Thanks so much.

Michael Olexo