Why do you have your cache on 4.2 ? O well dont know much about 5960.
But i tried to get my cache up as well. It seems very sensitive and doesnt like to be overlocked. Far is i could read you dont gain on overlocking the cache.
when i was overclocking my cache sometime it ran at 4.4 and past all test. But after a reboot or in games it crashed hard. I just accepted that by cache will not go over 4.1
Any ways thats my experience.
Try more, Try harder, Try and previal