Under "System Properties" my processor is identifed as: intel(R) Pentium(R) M
processor 1.60GHz

the next line VARIES, i.e. sometimes it indicates "1.60 GHz" as would be expected, but at other time I see 590 MB and other values less than the 1.60GHz.

The laptop is a HP Pavilion dv1420us, all original.

Maybe I simply haven't been paying attention [?], but I am not aware of seeing anything like this previously on any of my other computers.

The M stands for Mobile. Your system will vary the CPU speed in order to reduce heat output and to prolong battery life, even when on mains adapter.

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