|When that happens it's usually the PSU, motherboard or both (both as sometimes a bad PSU will ruin the motherboard). Since you've swapped in a (presumed) compatible power supply and had the same results the likely culprit is the motherboard.|
Well, less likely but still possible is the cpu or its heatsink assembly. You might swap in a compatible cpu if possible. And verify the heatsink is firmly attached and its fan is freely spinning.