PC shuts down and will no longer power on unless the CMOS battery is re-seated.

Here's the specs of my PC.
casing: Dell Optiplex 790
CPU: i7 2600
GPU: Radeon HD 6700
Memory: 2x 4GB ddr3
Mobo: stock mobo for Dell Optiplex
HDD: 1 TB SATA(don't know the brand)
PSU: 700 watts generic
OS: Windows 7 Pro 64 bit
others are just the basic stock fans/heat sink from Dell Optiplex 790.

This PC is more like a hand me down from a friend.
I left the PC for about 4 months with no activity since my monitor died and I still don't have money to get one. I recently bought a 32' LED TV to use as the monitor(which i'm now regretting). When I powered it on, I was able to play for around 5 hours until I decided to stop since the TV is starting to hurt my eyes.. Later on when I turned back on the PC, it was still fine and after around 2 hours of playing, it suddenly powers off and I can't turn it back on. I thought maybe the PC is overheated since I haven't replaced the thermal paste for a long time. I bought a thermal paste and replaced it the next day and it still won't power on. Then I decided to replace the CMOS battery and now the PC is working again. I thought that was the end of it but after a few hours of gaming, the PC powers off again, The same thing happened, I can't turn the PC on. I decided to test the CMOS battery if it was discharged but when I test it, it still has some life in it. I placed the CMOS battery back and now the PC turns back on again. I had to change the date and time each time I re-seated the CMOS battery. Again, after playing around 2 hours, the PC turns off again. I re-seated the CMOS battery and it's now turning on again. I decided to shut it down and find out what's causing this issue.
now for the question:
1. Since I'm using a generic power supply, is it possible that the PSU damaged my mobo?
2. Is my mobo dying? (it's really hard to find a 1155 socket mobo replacement here at our country).
3. What other possible reason that can cause this issue?

thanks.
Reply to legonaidas
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