In the end I got rid of the cheap unbranded DP cable, went back to HDMI and things have been average since then. Average in the sense that booting still doesn't seem normal and is almost like a double boot.
Incidentally, I bought Homefront Revolution is a Steam sale which then promptly messed my system up and the GPU drivers simply disappeared leaving me with basic. I reinstalled the drivers with DDU and the same thing happened, so I uninstalled/refunded the game and the problem hasn't returned. That was an odd one.
Anyway, I'm changing my CPU/mobo next month, together with a new high quality DP cable, so it will be interesting to see the improvements.