That and sometimes the limit the board can handle is what is already there. Had that prob on my old Aptiva, board could take up to a 475 K6-2, and thats what was in there...
If you know the motherboard manufacturer and model, you could try using software like SetFSB to raise your fsb, but could your stock cooler take that, not to mention the mobo?
Crashing takes a whole new meaning at 9.8 m/s.