Overclocking is very much the luck of the draw. There is no guarantee that you will be able to achieve 1150MHz on the stock voltage. Some cards can clock pretty high before you need to bump up the voltage to maintain stability. Other cards can barely get anywhere above stock clocks before they start crashing. You have one of the latter.
If you want to push further than 1025MHz, you'll have to increase your voltage. You have to do this at very small increments, and only just enough to get you stable at a desired overclock. Pushing the voltage too high will fry your card, so be sure to look up the maximum voltage spec for your particular card before trying to push it.