OSX Permissions Repair Help

Hi all,
I'm new to macs so some help/advice would be much appreciated.
Just installed 10.5.6 on my mac, done the 10.5.8 combo update and now my fans are on maximum rpm continuously, then after a while mac freezes.
Only way to unfreeze is a hard shutdown/reset hold power button for 5 seconds.
After doing a permissions repair all is ok for an hour or two then it starts over again
I have a G5 powerpc 2.7 dp, 3.5 Gb Ram, 500 GB HDD. (old i no but still a very capable machine)

Thanks' for you're time input

This sadly sounds like it could be a logic board error.
Do you have access to Apple Hardware Test?
I would suggest running it to see what shows up, bt G5's do suffer with logic board failure and the "fans of doom" are a good indicator.

Thanks Kevriano,

Do you or anyone no where can i get apple hardware test from Please

Or any other ideas thanks

You should have it on your install discs>

Apple Hardware Test (AHT) contains a suite of diagnostics that will test the hardware of your computer. It's is a great way to rule out a hardware issue when troubleshooting your computer.

* For Intel-based Apple computers shipping with Mac OS X v10.5.4 or earlier, Apple Hardware Test is located on the Mac OS X Install Disc 1 and should be included with your computer.
* For Intel-based Apple computers shipping with Mac OS X v10.5.5 or later, Apple Hardware Test is located on the Applications Install Disc 2 and should be included with your computer.

Edit. Completely forgot your on G5. As Harry has said, it should have come on a disc with your original install discs.

Apple Hardware Test came with the original install discs that were supplied by Apple when the machine was new. Apple store may be able to assist if you contact hem with model and serial numbers, but would not bet on that.

Thanks guys but i do not have original disks. (Bought mac second hand)

Looks like a long boring search on google to locate the right disks is my only option.

Thanks for your help people.

You could look for TechTool Deluxe for your OS, as it's not machine specific and has a hardware test. It comes with Applecare when you buy it.
Not sure if I still have it here, used to have. Will check tomorrow, if I do you're welcome to it.