Reduced CPU speed after bootup

Lately I noticed my computer running slower, and checked out the system properties. I have a 1.6 Ghz Pentium M processor, but the system properties showed that it was only running 589 mhz even on AC. I've monitored the speed, and found even if I get the CPU up to 100% capacity, it will still only run at 589 mhz.

 

Usually if I restart the system, then the CPU speed registers at 1.6 Ghz (almost) in the system's properties, and the CPU speed fluctuates according to the program needs as it is supposed to.

 

Why does the system sometimes refuse to go any faster than 589 mhz when it has a 1.6 Ghz processor? Is something going wrong?

 

D800, 1.6GHz Pentium M Processor, 1.0Gb RAM, Running the Precision M60 Bios (due to mother-board replacement).