try using the display driver uninstaller here Display Driver Uninstaller Download version 220.127.116.11
Then reset your monitor to defaults using its on panel menu. Then power cycle your monitor and restart
your pc and install new drivers buy re downloading them to avoid corruption.
make sure you have your monitor's driver installed. You can download it from here: EX3203R Software - Support | BenQ USA
From Benq's Resolution file:
I have attached Benq's Resolution file to this reply for you to download if interested.
So this didn't seem to work. I dropped it to 8 bit with dithering from the Monitor tab in the Radeon driver software (and rebooted) and still max hz was 120. Also tried a full uninstall using the AMD Software remover and did a clean install of the 4.1 drivers and oddly enough it changed the monitor to 10 bit again which is odd because I'm pretty sure its a 8 bit panel according to BenQ