well increasing the clock, when you reach a certain level sometimes you have to bump up the voltage. at what speed? well there is no one answer to that as each chip differs, even those of the same model.
usually, when it is unstable, it usually means its not getting enough juice.
safe temp, safe voltage, you will have to google a bit, it differs
You need to bump up the voltage once your overclock becomes unstable. You can do that or tone down the OC.
I would try to stay under 85c, and definitely stay below 90-95. That shouldn't be a problem with your gpu. My 7950 runs under 70 in most games with an 18% oc.