If all you did was swap out
CPU's then it is mostlikely a heat problem. What are your temps? What kind of
HSF are you useing. If it is an AXIA T-Bird you should be able to get 1.4GHz with good air cooling.
As for your MB. I have an Iwill KK266 which is almost a clone of your board. Those KT7a were some of the best KT133A chipset based boards. They should be able to hit 145MHz FSB without a hiccup. So I don't think it's your MoBo.
Since you had a Duron in there befor, did you also have PC100
RAM. Maybe it's your
RAM. Is it generic or is it good quality stuff? You may want to try backing off your memory timings.
And your
PSU should be up to the task. I have a 1GHz T Bird @ 1.4GHz @ 1.85v, A 7200RPM HDD, CD
ROM, CD-RW, Radeon 8500, a delta 68cfm screammer on the
CPU, and 4 80mm case fans. I'm sure I am forgetting something else but anyway. It is all on an Antec 300w
PSU and I have never had a power related problem.
I still think it is most likely heat.