One thing that would cause it to drop to ~1000MHz is if Cool 'n Quiet is enabled. It drops the CPU's speed and voltage when the CPU is lightly loaded, to reduce heat production, and thus cooling demands, and thus the amount of fan noise (potentially).
If you like, try disabling Cool 'n Quiet in the K8N's BIOS menu. It should be located under Advanced > CPU Configuration > Cool 'n Quiet Configuration, page 2-15 and 2-16 of the manual show where.