Odd progressive crash

I'm constantly getting this weird 'progressive crash'. It starts with
the air****t network dropping out (the other Mac doesn't even notice, so
the network itself is fine). 

The air****t icon greys out, but everything else seems normal. At least,
at first it does. I can switch apps - and use them - as long as I can
click their windows on the desktop. 

If I mouse over the menubar extras (by the clock), they work once - eg
they'll drop down a menu - then they beachball. The dock acts the same.
Then, a few moments later, the whole machine beachballs, and only a
forced restart (holding down the power button) seems to fix it.
Tem****arily. Pressing the power button brings up the shutdown dialog,
but it doesn't work.

I have just restarted a few minutes ago, and it's done it again whilst
writing this post. 

Any ideas?

        -zoara-


-- 
"Ignorance more frequently begets confidence than does knowledge."
                                     - Charles Darwin