Just curious...fsb limited to 240?

Using an older AMD Athlon X2 6000+ on an ASUS M2N-e mobo. (using one of those shmancy zalman fans + arctic silver, not the stock equipment)

Default fsb is 200 MHz with 15x mult.

Increasing my fsb to 250 renders the system inoperable (tried different mults from 1x to 12x; lowering HT did nothing) - it won't even boot into the BIOS post screen. Perfectly stable at 240, but going higher results in system crashes/freezes/failure of prime95 within minutes to the hour.

I've had a friend oc his X2 6000 fsb to ~270 @ 12x, so I was wondering: what might be the cause of this limitation?