I've had poor results with overclocking the 4 GB Sapphire Nitro+ 480s. I believe they have slower RAM than the 8 GB version though. Is it stock 2000 Mhz on the 8 GB version ?
I don't think you will be able to overclock to 2100 - maybe try stock setting and see if it is more stable.

I would try a slower GPU clock setting and lower GPU and RAM voltage - I think I had mine running at:

-cclock 1150 -mclock 1850 -mvddc 950 -cvddc 950

Try yours at -mclock 2000 and work from there.