What causes this to happen?!

[Allestaria]

All of my screens will do this after some play time. Can be 30 minutes or even up to 3 hours of play. I have to shut down and restart. Its not
always yellow. It varies in color. One screen will lock up and I have to ctl alt del to close it . I have close the other wow session from the tool bar.
My computer bogs way down when this happens also.
Hubby keeps thinking my video card is over heating. But it never seems to go above 60 Celsius.

FYI It doesn't matter if I go into a city. Though it does seem to happen more often when I do. The above happened after I took each one
of my toons solo into Stormwind. I figured that would keep it from happening but it didn't. I do get a bit of warning before it happens. My
screen starts to flash. Then the odd shapes will start to appear on random screens. If I don't close them the above happens.

Any idea's?