What counts as a stable overclock

[Zairs]

Trying to get my I5 760 stable, seems to keep crashing around the 3 and a half hour mark on prime.
I'm doing a custom test using 3600 ram.
The setting I'm testing at the moment are
21 x 199
1.375v v core
1.16875 imc
Ram is unclocked to 1200mhz
2.15hrs on testing so far

 

[Zairs]

I did have a higher clock will lower voltage for about 6 months and it never crashed, but I think I'm testing it more this time round

 

[Zairs]

I'm up to 7.5hrs