If you have tested the power supply on another machine (or replaced it recently with a known working one) a common problem for motherboards is that some manufacturers use generic Chinese capacitors in order to lower production costs.
The problem is that these cheap parts have a very short life and often after a year or so of use (sometimes less) the capacitors stop functioning properly which can cause the issue you are describing, in addition to random power downs, or sometimes the motherboards will not start at all.
Your best bet would be to Remove all of your devices except Motherboard/CPU/RAM and verify that the problem still exists with a known working Power supply. If you are still having issues I would just replace the motherboard, replacing all of the capacitors on the motherboard would be very time consuming, in addition to some motherboard have multi-layer boards which are virtually impossible to be serviced by the end user.
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If you have tested the power supply on another machine (or replaced it recently with a known working one) a common problem for motherboards is that some manufacturers use generic Chinese capacitors in order to lower production costs.
The problem is that these cheap parts have a very short life and often after a year or so of use (sometimes less) the capacitors stop functioning properly which can cause the issue you are describing, in addition to random power downs, or sometimes the motherboards will not start at all.
Your best bet would be to Remove all of your devices except Motherboard/CPU/RAM and verify that the problem still exists with a known working Power supply. If you are still having issues I would just replace the motherboard, replacing all of the capacitors on the motherboard would be very time consuming, in addition to some motherboard have multi-layer boards which are virtually impossible to be serviced by the end user.