Random White / Grey Screen and occasionally Black

Apple iMac "Core i5" 3.1 27-Inch (Mid-2011) - Running macOS High Sierra version 10.13.6.

Mac boots up fine and everything is working as normal, can be surfing the net or doing some work but then a good few weeks ago the screen started to randomly go a White / Grey colour (Does not happen all the time, and nothing in particular seems to trigger it), then a few days ago the screen started turning Black at random times and would reboot itself.

Have replaced the ram to see if that may have been causing the problem but unfortunately it is still happening. not really sure what else to try.

Any help would be appreciated

Thank You