If you are crashing in games, and its more than one game then it is very likely that you are not stable. Note that a higher GPU clock doesn't necessarily mean higher fps, I would run a benchmark like unigine heaven and watch the score and fps rates starting from stock clocks. Run the benchmark at stock, record the score, and increase the core clock and rerun the benchmark. If your score is higher and everything is stable, then increase the core clock and rerun the benchmark. If you see that you increased your clock and the score has dropped then you should rerun the benchmark again and see if it happens again. If the score is still lower then that would mean that you should lower the clock a bit and rerun. Sometimes increasing the GPU clock too far may actually reduce performance, and it would also cause the stability issues you are seeing.
On another note make sure you have the latest drivers as they may offer improvements for vsync.
Hope this helps!