ES chips are often early makes of the particular chip. Sometimes they clock higher than all other chips, sometimes they won't clock well at all. The major draw to ES chips is the unlocked multi, but now with certain steppings having unlocked multis (Intel Prescott's) it really doesn't matter with these chips. I would certainly rule out memory first, run a 5:4, or whatever it takes to keep the memory at specs (loose timings, divider). If you have decent air, go ahead and bump the voltage up. 1.55v actual is alright for these chips. What PSU do you have, how's the 12v rail? These Prescott's are power hungry, and need to be kept cool, what are your temps/cooling?