|You have a ghost using your PC when you aren't around. |
Seriously, it does sound like someone is changing the resolution. Is it possible someone else is using the PC?
Games sometimes reset resolutions.
If the resolution was falling back to default settings that would be different.
Try using a new password for login.
If the above is not a viable explanation then try updating the graphics drivers and installing any drivers that came with the monitor.
Also, verify the graphics card is fully seated.
You are clicking on apply then OK when selecting a new resolution?