To reduce the speed of GPU and vRAM can solve several issue for various applications and circumstances.
Such issues often occur with retailer GPUs, who overclocking the card above the standard OEM (manufacturer) values.
However, there is no recipe to solve all your issues.
If you're doing trial&error underclocking to stabilize the GPU, start by comparing the clocks of your GPU with the original OEM hardware (see Nvidia/AMD page) to see the difference between your current clocks and the official Nvidia/AMD values.
Important: Always start with the vRAM to over/underclock. Take steps like 5-15 MHz and do a stress test.
Once you're satisfied with the stress test results, take a note of the vRAM clock rate and reset it to default.
Then Repeat these steps with your GPU clock
Once you're satisfied set the vRAM clock to you previously noted stable values and do 1 or 2 stress tests with both combined.
A good basic introduction with pictures you may review here:
http://www.pcgamer.com/how-to-overclock-your-graphics-card/#page-2
Cheerio & Happy New Year,
Matricus
