Troubleshooting a dark PC display problem

I have a 6-year-old Dell Inspiron 3670 (500 Gb Samsung Evo SSD, 1 Tb internal hard drive, 8 Gb RAM) running Windows 10 connected to a P2219H Dell monitor. Recently the display started to fail (see my thread, "Problem installing driver for a dark monitor" in the Monitor forum). I had already eliminated the monitor or the cables as the problem. It appeared to be an integrated UHD 630 graphics problem, since after installing a Radeon 6450 graphics card the display started working normally and did for about 6 weeks or so. Then the problem of the monitor blinking off returned. It can go dark for a long period, or can start to blink on and off repeatedly. I now suspect either a power supply or motherboard problem. I can purchase an inexpensive power supply tester to check the power supply, but how do I go about confirming that the motherboard might be the issue? I could call Dell to see if they offer any motherboard testing services, which I know I would need to pay for. Are there any other approaches to testing the motherboard that don't require a lot of sophisticated equipment? Thank you for any advice.