Lock ups since CPU / Bus speed upgrade

Last Feb, I purchased Athlon Thunderbird 1200 100MHZ front bus speed with Iwill KA266R motherboard and 256 Megs PC2100 DDR Ram. The T'bird 1200 133MHZ front bus CPU wasn't yet available, but I intended the rest of the system to be forward compatible for a future upgrade.

Last week, I added an Athlon XP1800+, (133MHZ front bus) updated BIOS, and did a jumper change on the moptherboard to enable the 133MHZ bus speed.

Ecer since, my system has been unstable, with lock ups that are becoming more and more frequent.

I don't beleive I have a heat problem, as Iwill system monitor software indicates CPU is running no warmer than 54 degrees Celcius.

In my efforts to trouble shoot, I have changed the system bus speed back to 100 MHZ, and stabiltiy seems to return.

However, this isn't an answer for me as it defeats the purpose of my upgrade.

Any thoughts as to what may be the problem? Is my Motherboard hosed?

Thanks