Well, it would help if you could supply me with the name of the GPU you're using.
Also, each individual card has its own limits, due to variances during manufacturing. A voltage increase can help stabilize your overclock, but can decrease maximum lifespan, or at worst make your card unusable. I advise not to do that unless you're absolutely sure what you are doing
Try to keep that core overclock, but revert memory speed to stock. Memory overclocks don't do that much, unless the core is starved for data. Also, some manufacturers put memory on their cards that can barely handle stock settings.