another thing that comes to my mind is, that there are different cores of Athlon XPs. Perhaps you already know that, but in case you don't:
1. first Athlon XPs were Palominos
- close L1 Bridges to modify Multiplier in BIOS
- overclock horribly bad
2. then came Thoroughbred-A
- wire mod works here
- still overclock bad
3. Thouroughbred-B
3 different versions:
a) - wire mod working
- overclock differently, one has to have luck with the stepping
b) - completely unlocked!
- overclock generally good
c) - "Superlocked" -> wire mod didn't work anymore just as closing L1 Bridges or editing L3 Bridges
- mobile Mod does the trick (especially on SIS chipsets, for you have one)
- overclock without problems (it seems that they are downclocked by AMD)
4. same goes for Bartons
5. surely last Athlon XP is Thorton
- Superlocked, mobile mod again
- overclock extremely good (perhaps downclocked)
... they call it a royale with cheese ...