4-core slow due to disk access

[Joep L. Blom]

I have in Karmic (2-6.31-20) a nasty problem for which I cannot find a 
solution.
My system: AMD Phenom II 4 -core 4 GB of memory, 1 HD 250 GB and 2 
external USB disks (resp 250GB and 1 TB), which are irrelevant for the 
problem. I have an 8 GB swap-space.
After starting the system it's speed is acceptable but not superfast. I 
have e few applications running (terminal, Thunderbird, Firefox and 
gkrellm). I see in gkrellm that I have now 2 GB free and and ~ 1 GB of 
swap-space in use.
Normally I have vmware running with Windows XP but I have that 
temporarily closed.
I can see that I use from 1-5 % processortime (over 4 cores), 7 users (I 
have a few terminals started) and 447 processes.
But when I start a program it takes even > 1q second to start calculator 
and switching folders in Thunderbird, takes sometimes minutes.
In the gkrellm monitor I see a constant disk use for what?? (I have 2 Gb 
free). I cannot imagine that the system is constantly swapping.
I have this problem since I started the system (Aug-09) and the slowing 
progresses over time so I need to restart at least every 10 days,
Has anybody ideas and will it be better with 10.4 LTS?
Joep