Currently Being ModeratedApr 29, 2012 11:49 AM (in response to Brill Phady)
I would recommend running Apple Hardware Test in Extended Mode, if my hunch is correct it's going to find graphics card issues. The only fix for that is replacing the card. Depending on the age of the computer you may want to consider replacing the machine. Based on your running Tiger 10.4.11 the machine is probably 5 or more years old and about due to be replaced.
Currently Being ModeratedApr 29, 2012 1:07 PM (in response to Brill Phady)
OK, a couple of things about AHT, first you have to run it in Extended Mode. Next is it's not the most reliable test so I'd run it at least 3 times to give it a chance. If it still doesn't report errors then take it into an Apple Store or AASP for professional diagnosis.
Currently Being ModeratedApr 29, 2012 2:12 PM (in response to rkaufmann87)
How do I run in extended mode?
Model Name: iMac
Model Identifier: iMac5,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.16 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache (per processor): 4 MB
Memory: 2.5 GB
Bus Speed: 667 MHz
Currently Being ModeratedApr 29, 2012 3:03 PM (in response to Brill Phady)
Please watch this video, when it gets to the 1 minute 30 second mark you will see a box that says Perform Extended Testing, check it!
Currently Being ModeratedApr 30, 2012 2:05 AM (in response to Brill Phady)
Some of the 2006 iMac line were affected by LCD Display issues where lines (solid coloured lines, some green, blue, red etc) started appearing on the screen. Apple did have a repair program up and running for affected machines, but I think this program has now ceased.
You said you installed software to speed up the fans which seems to be working. This sounds like a heat issue to me. Have you tried using a vacuum cleaner (with a hose) to clear the air vents / ports etc on your iMac?
If you switch off and unplug your iMac / peripherals then remove the memory panel and carefully vacuum out any dust that might be in there and around the ports at the back, it might help with the air circulation and resolve the issue (if it's the GPU overheating).
It could also be indications your Graphics Card is starting to give up, which could be a costly repair.