The desktop (tower) PC has been suddenly 'dying' without notice, after varying numbers of minutes. The CPU heatsink fan stops turning, but a LED on the motherboard stays lit. I experimented with the RAM sims and slots, but got very inconsistent results. I tried to run MEMTEST to get a 'sophisticated' evaluation of the RAM, but the PC kept dying before the test ended. I thought I had heard some 'arcing' noise, so my next suspect was power supply. A replacement 'fixed the problem' ONLY while the PC was left on its side; when it was turned upright - as a tower is supposed to be - the problem returned. Any suggestions what loose connection to look for ?