I am still running v26, which appears very stable. It went over 4000 accepts on each rig with less than 1% rejects. --scryptr
I agree, v26 was also very stable for me. 11540 +/- 50 khash/s, accepted: 11422/11476 (99.53%) for two EVGA 750Ti SCs.
I just built v28. A little improvement for Quark, 11680 +/- 50 khash/s.