I see you have a 2013 iMac. Coloured square dots and crashes seem problem with iMac 2010 and 2011 with AMD Radeon HD 6750M and alike card in MacBook Pro and iMac.
I check the temperature too, and never had over heating. I use iStat widget to monitoring the GPU heatsink and Techtool Pro to have diagnostic of whole system. For example, a few minutes ago, an HD video running to keep the GPU hot, I pasted the result "à la fin" of this post. As you can see, temperatures are in the normal range.
I think you are a lucky one :-) to have a problem you can resolve. Probably not the same as mine, unfortunately.
One more thing, as I replaced the original drive of 500 GB, there is no more thermal probe, so I installed "HDD Fan Control" to control the ventilator. It works fine.
Merci to share your observation. But I continue to think of a AMD pizza :-)
Alex
From Techtool Pro "Sensor test" ;
Ambient Incoming Air 26.73 °C
CPU Heatsink 46.88 °C
CPU Proximity 50.88 °C
CPU Proximity 0 50.88 °C
GPU Die 61.00 °C
GPU Heatsink 59.75 °C
Hard Disk Proximity 9.00 °C
LCD Proximity 42.28 °C
LCD Thermal Compensation 42.83 °C
LCD Proximity 0 54.94 °C
LCD Thermal Compensation 1 44.14 °C
Optical Drive Proximity 46.00 °C
Platform Controller Hub Die 61.00 °C
Skin Upper Right 43.25 °C
Skin Upper Left 44.25 °C
Main Logic Board Proximity 56.62 °C
Power Board Proximity 68.12 °C
Power Secondary Heatsink 79.62 °C
Power Primary Heatsink 59.25 °C
