It could indeed be a hardware issue since most of the times these heatsinks are loose, affecting cooling (bad cooling equals less headroom for OC'ing).  Also, some cards being not seated well can also affect cooling, same result.  Lastly, not fully seated cards can have poor connections with all sorts of problems leading to poor OCability (heat, electrical noise, etc).

So, yes, open it.  Carefully tighten all heatsink screws and seat the cards well and evenly, and then carefully reassemble (the LED and USB connector have to be watched closely when putting the face plate on).

If that doesn't fix your OC problems, be sure the ambient temperature is not too warm and you have good airflow around the devices AND that your powersupply is not the problem (numbers aren't the full story here, also has to do with the surge control, the rail stability, etc).