Why does my system stop frequently?

[Dennis Wicks <wix@mgssub.com>]

Greetings;

My system (lenny 2.6.26-2-686, P2, 2+ GHz) will just stop. 
No disk light, nothing works. No scrolling, typing, 
marking/selecting, no copying/pasting. Just frozen. It will 
stay like this for probably 5 secs. although it seems 
longer, and it happens often, every couple of minutes.

It will also run really slow. Steady disk light and although 
I can switch (slowly) between workspaces, nothing else can 
be done.

Any help will be greatly appreciated!

Many TIA!
Dennis