This is a sign that the graphics card is not happy.
There are several reasons why:
Overheating
Damaged/Faulty GDDR memory
Insufficient power
Glitch fixable by a patch or driver
Overheating, is fixed by ensuring there is no overclock and your case is well ventilated with fans and dust free. If it is summer, point a fan at the computer with its side off and see if that helps. Remove all dust with a can of compressed air (hoovers can be used but can also kill equimpment so watch the static and all care no responsibility

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Sometimes graphics cards go bad and commonly the RAM component (GDDR) can cause the graphical annomalies you describe. In this case the only way to know is to try the card in another computer with sufficient power supply. If found faulty you need a new graphics card.
The power supply unit (PSU) is the most important part of a computer providing clean and reliably powerful current to the graphics card. If you have a cheap or underpowered/poor quality, you can see these graphics issues. Insufficient power causes resistance and heat, causing similar symptoms to overheating. This can also kill the RAM and the card.
Try the above cleaning and cooling measures and then download PC Wizard from my sig and install it.
Go to FILE, SAVE AS and click OK, then copy the text out of that file into this thread.
Look inside your computer and record the model and brand of your power supply unit (the power box inside computer). Dont be alarmed, take normal precutions (its only 12VDC inside a computer).
Post the PC WIzard report and the PSU info and let us know if anything has improved with cooling.