Computer stalls

[rh]

I have two problems with my Ubuntu box.
Fist one is, that sometimes there are continued disk ascesses for a long
time (this may take ten or twenty minutes) whilst the computer is almost
dead. I have tried to look with "top" if there are any processes which
takes an unusual amount of time, buit i could not find anything.

The second thing is that often there are very large Java scrips working
in Firefox which paralyze the computer. Once I tried to download such a
script. It was a huge unformated JS-file, unpossible to read or
understand what it does.

Has anyone an idea how I can find the cause of these Problems?

-- 
Reinhard