Is my motherboard going bad?

My computer is 8 years old, but does what I need it to do. My hardware is a Gigabyte EP45-ID#P mb, Intel core 2 duo 8400, Corsair TX 650 power supply, and because I thought it was my graphics card, a new MSI Geforce 950. Memory is G Skill DDR2 1088 which test as good. When I power on my computer, I see the posting screen, and sometimes, get to the welcome to windows screen before the monitor goes blank for around 3 minutes. Then the monitor pops on, and off, a few times, then works perfectly. The monitor works if I use it for streaming TV, I tried a new cable to the monitor, I had the same problem with the old, and new graphics card. A software program tells me the voltages to the mb are good, and all the temps are excellent. I have a power supply tester on order. To rule out viruses, or software problems, I installed a new hard drive and did a clean install of Win 7. I know it is time for a new build, and I am saving up for that, but I would really like to identify the problem I have.

I see nothing visible from the side accessible, but have not pulled it yet. I have a case with excellent cooling, and have never had heat problems I am really puzzled by the short "warm up" time, then everything works. It works until the monitor is put to sleep, then it begins again. I found a deal on a used mb, cpu, memory combo, and will switch it all out next week. That should confirm if it is the power supply, and give me an opportunity to look at the other side of the mb. It has been doing this for a couple of months with no apparent change. I know all things wear out, but not being able to pin point the problem is annoying. I am shocked at the price of an exact replacement mb. They have held their value quite well.