Try higher FSB (in BIOS or with jumpers). If it doesn't works cover pin b21.
If it is PII 66MHz FSB should ran at 100MHz FSB.
Go step by step 75MHz FSB(75*4.5=337,5),next 84MHz FSB(84*4,5=378),95MHz FSB (95*4,5=427,5)
and 100MHz FSB (100*4,5=450). You can try higher FSB than 100 but I have never...