The fact that the monitor works sometimes and not others makes me suspect the monitor drivers. Try uninstalling/reinstalling them:
# Open Device Manager by typing ***devmgmt.msc*** in the Search Box in the Taskbar at the bottom of your screen.
# In the Device Manager window, scroll down to the entry for "Monitors." Click it to expand the list of drivers.
# Right click the drivers and select "Uninstall" from the popup menu. Then, restart your computer. Windows will automatically detect your monitor and install the drivers it recommendeds.