Very bad performance when using battery on my laptop with FB7

[Kemian Dang]

Dear all,

I have a Compaq laptop running FB7 stable.
When I use battery, the system becomes very slow, there are even lag
of the cursor.
The "load average" would be 1 - 2, while the idle CPU is 99.0%.
It would not change even I plug the ac power later.

But if I reboot the machine whit ac power, everything would be OK.

Any idea appreciated for what should I do to solve this ...

Best wishes,
Kemian