As far as I know increasing frequencies (without tweaking voltages) still has an impact on power draw. If you detail what what hardware you have someone might be able to tell you if power draw is the problem.
Also if your gpu overclock is stable when doing furmark then what are you doing when you're computer freezes? Gaming? You may have issues with your overclock only when the cpu is working hard as well. I suggest running prime 95 (to stress the cpu) and furmark at the same time to see if this causes your instability issues (but keep an eye on temperatures). Just leave one thread of your processor free for furmark (eg I have a quad core with hyperthreading = 8 threads. I would run 7 threads of prime95 and leave the last one for furmark).