Don't change core and memory clocks at the same time. First OC the core and test for stability. Then overclock the memory and test for stability. You can also use HWiNFO64 to watch for memory errors on the graphics card. Although there's some debate as to how many (if any) errors are acceptable to still call the OC 'stable'.
Given how mild the core OC is I would guess that it's your memory OC that's the issue if you're crashing, but that's just an educated guess. But yes, in general if your core clock isn't stable you can add more voltage to stabilize it. You could try to do the same thing for memory clock/memory voltage, but I'm less sure about what exactly the "memory voltage" setting in Wattman is and what effects changing it has, so I've never played around with it.