Whenever I try to start my pc after it has been off for a while, I get a blinking power light and it won't start. If I unplug the power cable and plug it back in, it will then start up most of the time. Does this indicate my power supply or motherboard is going bad? Other than this starting issue, the pc seems fine.

Seems to be your power supply.

Have you tried with a different power cable to see if it's the problem before checking the power supply