Than I'm stumped. Sorry. Maybe someone else here has an idea. If you're got a support contract with MS, consider logging a case with PSS.
I've had very bad experiences with drivers for the AS400 on an IA64 box, but SNAC in x86 should be fairly well behaved.
What you could try is logging the results of xp_memory_size at regular intervals after a server restart. If you can find out when the memory gets fragmented, maybe you can figure out what's causing it.
Maybe unrelated, but if your max and min memory settings are the same, maybe reduce the min by a couple MB. I've heard (but it may just be urban legend) that setting max and min memory to the same value stops a couple of memory management routines from running.
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