My bet is still on nothing wrong being with the Bitcoin-Qt task but some general computer problem, e.g. failing disk or disk controller, bad non-ECC RAM, or one of those "power saving" graphic card issues when the computer has two graphic controllers (one Intel/AMD on the chipset for "low power", one discrete AMD/Nvidia for "high speed") that are supposed to serve same screen, but sometimes lose the mutual synchronization.
The fact that the problem tend to disappear when you minimize and restore the program window would tend to point at the graphic controller problem (hardware or software) e.g. one of those "Movie Color Enhancement" utilities that try to optimize color palettes in each window to make them look "prettier". IIRC 0.14.0 started using window composing within the Bitcoin-Qt window where it dims the most of the window and pops up other unmovable window in the center of main window to display the estimate of how much time is going to take to resynchronize the blockchain. Particularly dumb "Color Enhancement" utilities would then go to endless (or near endless) loop trying to optimize the appearance of Bitcoin-Qt window.