Low ambient temperature would not cause a BSOD issue, in fact it should actually assist with the cooling of your system and even in improving performance. 20-21C is not really low, I have used systems in cooler environments and they work marvelously.
Run the Intel® Processor Diagnostic Tool, this should properly diagnose your CPU and detect any problem, you can download it here http://www.intel.com/support/processors/sb/CS-031726.htm.
Diagnostic tool shows nothing errors.But no one knows what the problem is. And I changed the memory, graphics card, motherboard, hard drive and SSD. Even the power supply. But nothing helped.
It is improbable that this is a processor issue, since it passed the diagnostic tool and unlikely that this is related to low ambient temperatures.