Ok, stop fiddling just now, and read the following
as you overclock higher, the Cpu or in this case Gpu requires more voltage to accomplish the job, so you up the voltage to achieve higher stable overclocks,
however there are two main things to avoid, one is overheating, which although detrimental to your chip, you can monitor and react to,
the other is over the limit voltage which can fry and kill your chip in a microsecond,
before you overclock any chip you should find out its limits, at least the voltage, or you risk killing your expensive setup instantly
You'll have to find out what chip is in the card, check reviews etc for that, then try to find its V limit, google often helps here 'cayman voltage' for example or '7970 voltage'
I would also suggest reading through some overclocking guides as there are a few tips you can pick up on general practice, that apply to any chip being overclocked
You've been lucky so far but please research and learn a bit more before trying to push further