I'm running a P111 coppermine 1Ghz on cusl2-c MB. The cpu is continually reported as running at 500mhz, although diagnostics (sandra etc) show cpu range as 500-1000mhz. The bios settings have changed and I cant reset them to 1000mhz because on save & EXIT the bios just reappears saying that the settings are wrong. (have updated bios etc etc.)It will only work on a lower setting. This is causing me major probs with CAD apps. It was running at full speed  a while ago.
What can this be??

Try one thing. Take out the CMOS battery from the motherboard. It is about the size of penny and its colour is shiny silver. After taking it out, turn your PC ON and goto BIOS SETUP. Now exit wihtout save and turn your PC OFF. Now put the battery back into its orignal place. Also CHECK if the battery is slow, replace it with new one.
After putting it back, goto to BIOS setup and load BIOS's default settings and then 'save and exit".
I do hope that this will solve your problem.
D Wahab