Shutting down when 'cold'

My computer:

• iMac (27-inch Core i7, Mid 2011)
• Radeon HD 6970M (2 GB)
• OS X 10.10.1

I have a weird problem with my iMac. The machine crashes when it is cold.

So if the iMac has been sleeping it will crash almost always when waking up. Or when I boot the machine when it has been unused (cold), it will crash if I start a program ’too fast’. This period seems to vary, but three minutes is too little it to ’warm up’. And around 8 to 10 minutes it won’t crash when I start programs. Sometimes iMac crashes when booting to Os X too (if too ’cold’).

To avoid this I can push option-key (choose operating system menu) and let iMac to ’warm up’ some time. When using the iMac ’warm’ I can do anything and it doesn’t crash - gaming, photo editing, encode movies etc. I can also boot the machine without any problems if it is warm.

But here is a twist with this problem: Using bootcamp and Windows 7 the iMac doesnt crash EVER - cold or warm. Weird.

My guess is that there is something wrong with power unit, motherboard or display adapter. Maybe some component had too much thermal expansion somewhere? I would take this machine to service, but I am pretty sure they only charge me 70 euros for opening the machine and can’t do much. Replacing all three components seems too pricy too. Anyone else ever experienced this kind of problem?

Here is a list of things I have tried this far:

• Reinstalling Os X (cleang install). I have tried both 10.9 and 10.10.
• Hardware test (long or short) doesn’t find any problems.
• SMC reset.
• PRAM reset.
• I have tried using different memory chips. And also on different slots (the iMac has four).
• Have disabled wifi and bluetooth.
• Used different power cord and took computer to another room for another power socket.
• Used external firewire hard disk for Os X (10.6). Crashed also when ’cold’.
• Have connected three external usb harddrives when booting (’trying to overload faulty power unit’). No problem at boot.