Imac G5 keeps shutting down

I have an IMac G5 - late 2004 model. Running 10.5.8
I added extra ram a couple of years ago - nothing else added.
I've run disc tools and the report said everything was fine hardware wise.
I've reset the management system/PRAM.
I've run Disk Utility but the Permission Repair option just runs and runs (usually causing the computer to shut down).
The problem: I can leave the computer switched on (without using it) and it goes in and out off sleep mode no problem. However, once I start using it the computer can suddenly crash (not hang - the power just goes with a 'fizz'). It can happening opening a file (but not the next time you open the same file), using a program, browsing the net, copying files.

I can restart and things might be OK for half an hour/or a minute then shut down. The computer does not feel as if it is overheating. It seemed to be working perfectly until the Safari 5 download. I thought this was a hardware fault but now not sure - can anyone help?

Try installing the program "Temperature Monitor...and tell us what temps you get (after things warm up)...so that we can try to eliminate overheating as an issue.

Temperature Monitor

HTH,

- Nick

Ok I've installed the monitor, been on the web, opened programs, copied files, played video etc..CPU temp reads 63C but moves up to 64C max and down to 61C - when I'm using the machine. Disc at 41C. Ambient temp currently about 28C.

Just checked again - now down to 58C.

Ok computer just went off as I was playing a sound file - temperture was up to 70C.

Jennywren said:

Ok computer just went off as I was playing a sound file - temperture was up to 70C.

Click to expand...

Well its not over heat is about 140F so i do not think its over heat. Might just be a System Problem. Did you try Booting the Computer on a Another Disk or OS X install?

I've booted off the Tools CD and ran all the test no probs. I think in total the tests ran for 45 mins at least and the computer never crashed while running from the tools CD.

Jennywren said:

I've booted off the Tools CD and ran all the test no probs. I think in total the tests ran for 45 mins at least and the computer never crashed while running from the tools CD.

Click to expand...

You might have a bad Ramdisk/Swap File. Trying Doing a Permission repair with the Mac OS X Install CD. Along with a Disc Repair From the Mac OS X install CD. Then Again. Reset PRAM, And the SMC Chip. Then See if it Does the loopy stuff it has been doing... If It fails again try doing a OS X reinstall. Cuz from the Sounds of it, i think its just a software problems.

Booted from Install disc - can verifiy disc (says it's OK) permissions another matter - says it will take less than a minute but the indicater bar stays near the beginning and is just goes on and on....So thought I'd reinstall - AHHHH Start DVD holding down c (computer shuts down if you try and restart the install disc from exisiting desktop). As soon as install tries to verify DVD computer dies.

Allow disk utility about twenty minutes which is the norm. Install disc one that came with the computer when new or a full retail install DVD and not a silver grey model specific disc?

harryb2448 said:

Allow disk utility about twenty minutes which is the norm. Install disc one that came with the computer when new or a full retail install DVD and not a silver grey model specific disc?

Click to expand...

OK did that (with orginal Leopard DVD) - it listed lots of errors with the permissions and claimed to fix them. Also ran the other program and it said the disc was fine. Also, reran the orginal CD with hardwaretools on - this ran for over 25 mins without dying - checked the hardware and said things were OK. Makes no difference....restart Mac bring up the temperture monitor...if I run disc utilities to verify permissions I can watch the temperature rise to about 79/80 degrees celcius then 'fizz' no computer. The fan (I think it was the fan) sounds turbo charged when I run the computer from the hardware tools CD. Just about pulling my hair out here...

Fixed!

OK this turned out to be a heat problem. I took off the back of the machine and gave it a good clean with a Hoover! It has been on (and being used in ernest) for three days with no problems. Looks like dirt/dust was the problem - I think maybe the hot weather just pushed it over the edge.
Thanks to everyone who tried to help.
Jen

Jennywren said:

OK this turned out to be a heat problem. I took off the back of the machine and gave it a good clean with a Hoover! It has been on (and being used in ernest) for three days with no problems. Looks like dirt/dust was the problem - I think maybe the hot weather just pushed it over the edge.
Thanks to everyone who tried to help.
Jen

Click to expand...

Just curious...how dirty did the inside look, and how dirty were the fan's?

For example...was there just a light coating of dust...or was the dirt/dust/fuss really thick & "clumpy" in some areas?

Congrats on finding the problem.

Thanks,

- Nick