Hi,
Although I agree that RMA should be the thing to do, it does not hurt to look at the heatsink contact and, if too short or misapplied thermal compound, reapply since this is *probably* the real cause for those core underclockers.
In my case, my MSI 4200 did out of the box overclock very well (core up to 315). Wanting some more, I installed a TT GF4 cooler, and when removing the HSF, I saw there was *too much* thermal compound. I removed it and installed the new HSF with a very thin layer of ASII. What was the result ? The card would no longer overclock higher than 295. Reason ? The core was not perfectly flat, and with a thick layer of ASII, all of it was making contact. A thin layer left a big 'hole' in the middle without contact.
So, not wanting to risk a 200$ card (at that time) by lapping the core, I finally reapplied a thick layer of ASII, installed the TT FG4 cooler, and again good overclocks!!!
Regards
FTC