So you're having to reduce from the stock, factory clock speed? That sounds like a returnable issue to me. Voltage increase could solve, yes, just as with CPU overclocking, but the important part is your GPU should run at factory default settings correctly. If not, it seems like a potential indicator of future problems (i.e. problematic/flaky/out of spec hardware).'
Glad you got it resolved at least though.
- Oshyan