I have exactly the same problem, but perhaps of my own making.

I have an NF7S-v2, Barton 2500+ (but I select 3200+ ie run with the
200MHz FSB), 512Mb DDR 3200 Crucial (Non ECC, Non parity,
unbufferred), Maxtor SATA 160Gb HD, ATI 9800SE AIW, and a cheap 400+W
PSU (designed, but not approved for P4 with the secondary connector).

My only added observation is that when I do reboot and go back to the
BIOS the FSB speed has sometimes (perhaps always) been set down to
100MHz.

A friend mentioned that there was some trick with the reset switch,
pressing it three times in quick succession to lower the bus speed
when overclocking, could this be related?