There are a few things that can cause this behavior.
1. Overheating
2. Poor quality power
3. Failing graphics card
The first is the easiest to check, use software to monitor the GPU temperature while gaming. If it looks like it's getting above the mid 70 to low 80's, you may need to clean the dust and cruft out of it's heatsink.
If it's not overheating, try the card in a different computer, or try a different power supply in your computer. This is obviously more difficult.
The is the last resort and would require trying another graphics card in your computer.