Part black screen on start up

Hi I have a HP Pavillion 22XI desktop monitor connected to my Apple Mac Mini. The monitor i have had since 2013 from new. It has recently started displaying a part black screen on start up. If I then switch the monitor on and off about 10 to 15 times then it will show a proper full screen without any dark spots and will stay good the whole day until I switch off my computer and monitor and then it will do the same thing all over again on starting up and will only show a proper full screen again after switching the monitor on and off a few times again. I have done a factory reset for the monitor a few times and I have also replaced the HDMI cable but it makes no difference and have unplugged and cleaned the power cable as well but the problem remains. Help would be appreciated.