Some people run Prime 95 for at least 8 hours, claiming it gives the room time to heat up; what I did was run the high fft setting and blend setting for 8h+ each, the blend setting helps to see if there are any issues with overclocking your ram, while the high fft setting is for mostly cpu to generate the most heat.
Seeing as how your crashing stopped when you dialed it back a bit, it wasn't 100.00% stable.
If you plan on running it 24/7 at that speed (Or just whenever you have it on) and want to make sure its stable, try running the prime 95 for 24h straight, giving it time for any instabilities to come out into the open.
Edit: Be sure to monitor your max temperatures and make sure you aren't getting too high, and keep in mind, if you don't have adequate air flow in your case, your gpu will also increase the ambient temperature inside your case, which will affect the cooling ability of everything in there. (Just mentioning it)
Hope this helps.