Screen keeps going black for a couple of seconds then OK, then black again for couple seconds and returns again. Maybe it will be OK for 10 minutes maybe an hour.
One other odd thing is that when PC switched on and the windows is loading with the four colours coming into focus, 2 dotted lines appear, one after the other across the centre of the screen, then welcome screen appears.
This is all since new graphics card fitted by engineer/technician.
Any help would be greatly appreciated.