EDIT: first please answer the post of
menotu, maybe all things below aren't necessary!
First, Welcome to the Forum (post count of 1 indicates this

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The next time your laptop freezes, can you check if the Numlock light / Caps Lock light / other lights are blinking (light on, light off, time in between about 1 second)?
=> would indicate a kernel panic
Something else you could do is (just after a freeze):
Open a Konsole (I assume you're using KDE4, in the future please always say what you use, because there are so many possibilities), and become super user by:
su [ENTER]
and typing your password (you won't see any reaction on the screen while typing), following by [ENTER]
then:
tail -n 50 /var/log/messages [ENTER]
Copy that lines to a post here on the forum (please use the quote tags or the code tags for that)
Thanks
PS: I would put that modem led thing in a separate thread (one problem per thread, else we loose sight on things

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