I've bought a used Mac Mini, and there is a big problem with it. Sometimes the screen change as if a blue-red filter covers the screen. Check the picture to see what I mean. The mini is connected via a no-name HDMI cable to my 47 inch Phillips TV This annoying problem occurs at arbitrary times. Sometimes it happens at intervals of 5 minutes, sometimes 30 minutes. After about a minute or two the problem resolves itself. Alternatively i can resolve it myself by changing my resolution. The problem is unbearable and I would like to resolve it instead of contacting the seller to get my money back (he lives far away, and besides the problem, its a lovely machine). Do you guys know this problem? And do you know a solution? I've tried resetting the pram and formatting and reinstalling Yosemite - without result.