Unable to Overclock Core Offset

Ok, so i recently upgraded from a 550 ti(which was a temporary card until the 600 series came out and is now in my HTPC) to a MSI GTX 660 ti PE OC.

I bought the card on December 8, 2012 and the shipping company lost the card somewhere along the shipping process. Luckily Amazon resent another card using UPS and sucessfully reached my home. After opening the box, I discovered the card was a used or refurbished model based on the stains, dirt(not dust) and other unknown stuff on the PCB. Also the connectors showed signs of wear and use so it was evident. I didnt care too much since I cared more for the fact it works. After testing and making sure it ran, i was happy. A month and a half later, I am now ready to overclock my card to see how far it goes.

The issue that i ran into and i think its because its refurbished is the fact i cant increase the core offset more than +30 mhz in Afterburner without getting artifacts or Unigine Heaven to crash... I can increase my core to +50 mhz and furmark will be perfectly fine but Unigine crashes... I can increase the power limit to 114% and the memory +550 mhz without artifacts but when i even touch the core offset, it crashes. Plus, according to GPUZ, with core voltage increased by 100 mv or not, during Unigine Heaven my core voltage is 1.175v and doesnt change unless boost changes. Btw, my card boosts to about 1189 mhz whether power limit is 100% or 114% which is why i want to overclock.

I have no idea what to do and i have tried practically every setting afterburner has to offer with no avail... Is my card just not meant to overclock? could it be from being refurbished? should i try to return it or sell it to get another card?