Artifacting (screen corruption) at low temperatures is usually one of the symptoms of a card on its deathbed.
But before crying and starting to look for a replacement try these steps:
Remove and clean out the card, if you do n't know how to do this, post us, we'll give you instructions.
Update the video drivers.
If possible, through the drivers, set the fan to run at full speed, all the time, to keep the temperature down.
Your case:
http://www.overclockercafe.com/Reviews/cases/Tt_LANMOTO/
Has options for front and side fans, are they installed and blowing IN?
Check the CPU and motherboard coolers are also clear, Speedfan or Realtemp will tell you the CPU temperatures.