Someone is going to kill you for continuing to call the CPU Cache speed as the NB frequency. Lol.
Your core voltage is also your cache voltage. If you're crashing at higher cache speeds that means the core voltage needs to go up. 1.15v VCCIO/SA is standard unless pushing high DRAM speeds.
Sacrificing your cache speeds down so low for more core speed isn't the best idea, imo. I know a lot of people say it's useless to OC the cache, but balance is best in all things. I'd try to get stability at least at 4300 on the cache.