The motherboard would be the first suspect, but I've come across cases where the same symptoms were caused by different things. A defective CPU, PSU (SMPS) or a weak CMOS battery can definitely cause such symptoms.
In a few cases, one of the 20 pins of the ATX PSU connector did not make proper contact with the mobo. In the case of two newly assembled computers, one of the female points on the PSU's ATX connector had not been locked properly in the plastic housing, so that it did not make contact with the pin on the mobo.
In some other cases, one of the pins had been badly burnt, causing random crashes and ultimately POST failure. Some of those pins carry a heavy amperage, and a less than perfect contact can cause a lot misery to the user and to someone trying to diagnose the problem.
In yet another case, the owner had removed and reinstalled the CPU to test another CPU, and in the process he nicked one of the tiny copper tracks on the mobo. The break was almost invisible even with a magnifying glass.
That caused me a lot of headache because the original Duron 600 worked on another mobo, and the mobo worked with a Duron 1.1GHz and an Athlon XP !!! (I didn't have a spare Duron 600 for testing). Once the problem was isolated, it was a simple matter to bridge the break in the copper track with a tiny blob of solder.
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