Help With O C Intel D915gag

hey i gota pentium 4 3.2 and im having trouble overclocking my intel bored is locked but i got SETFSB software and i can overclock but i wanna overclcok 2 3.5 but i can only get it to 3.44 and if i go any higher it just freezes and i have to pull the plug how can i get pass that or is it the max i can get it to??? i will really like your help coz im new to this type of stuff