this normally happens when you start your computer with the monitor unplugged.
to be safe linux sees there is no monitor and so sets the resolution low so that any monitor plugged in will work.
but if you plug in your monitor and reboot (or maybe just restart the xserver) the resolution should go back to normal.
a reinstall is not the answer.
you may also be able to use the screen resolution app to change it.
I dont know how "smart" the auto screen resolution is. any more info you can give about the how and when would be helpful.