If you can't find the multiplier value in BIOS, run CPU-Z and something to get your processor at full speed, like Prime95. Look in CPU-Z for the GHZ the processor is running at and the multiplier value while Prime95 is running. But don't mess with the other stuff in BIOS.
CPU-Z:
http://www.cpuid.com/softwares/cpu-z.html
Prime95: http://www.mersenne.org/freesoft/
As an alternative, you may try resetting the BIOS to default and see if the speed returns to normal.