Did it occur to you that the bus speed might be set to 100MHz when it's supposed to be 133MHz? If it's dealer assembled, make HIM do the proper settings.
Also, you do know that the XP+ number is not a real MHz number? It's a PR number, like the old Cyrix PR-300 that only ran at something like 200MHz and was easily stomped by a Pentium 233...only in this case it's the PIII 1400 easily stomping the XP 1500+
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