other people with the same card have gone higher (1000/1150/1.3) and they don't have a problem
Not every card can overclock the same. For example, many people can push their 7970s well into the 1200 MHz range. Mine isn't stable an inch over 1150 regardless of voltages. That's just par for the course with overclocking - sometimes you get a great chip for overclocking, sometimes you don't.
My guess is that you just didn't get a card that has much OC headroom as others. You can only judge things by your own settings and stability, not what others have done.
:edit: One thought - your voltages might be too high to be stable. Assuming 1.17 is stock voltage, you shouldn't ever just jump your voltage setting up to the 1.25-1.3 range. Try to find your max. OC in OCCT without any voltage changes (probably could try 25-50 MHz increments from stock). Once you get to a clock that is unstable, increase the voltage by 10mV at a time (i.e., 1.17-->1.18) and see if you can get it stable. Once the new clock is stable, increase it again (10 MHz at a time) until you need more voltage (again, increasing voltage 10mV at a time). Repeat until your card generates too much heat or the OC becomes unstable regardless of voltages.
It's slow and painful at times, but it's far more reliable than "hey, I think I'll just max my voltages and then see if I can get 1 GHz to run... this one guy did it and it worked great!" [not saying this is what you did necessarily, but I've seen a LOT of people basically take this approach to OCing a GPU]