I have a 5 year old ViewSonic E790 19" display, and a Geforce Ti 4200 graphics card. I have updated all the drivers. Once in a while the display corrupts, showing patterns that look like large pixels. Sometimes it appears as a series of vertical lines, each 1/2 to 1" in width. Rebooting won't fix it, I need to power off the computer and power on to recover. Any thoughts as to what it is?