Did you overclock ram too? You cant just turn up the fsb you also need to turn up the ram freqency. just for reference I have E6600 and as well but with the Freezer 7 Pro cooler with Asus Striker Extreme board and also crappy 667 RAM. I get 3.4 Ghz up to stable but I keep at 3.33Ghz for safety purposes? Do you have aftermarket cooler or stock cooler.
Stock cooling will be able to run 280fsb fine. If your running 667mhz memory and you raise the fsb to 280 then your memory will try to run around 700mhz which might be a little to fast for stock volts. Try raising the memory voltages up a little bit. Or like itotallybelieveyou said to set the ram and fsb to 1:1 ratio which should work.