Gateway slowed down to barely usable

[Andrea Venturoli]

Hello.

A 6.1p13/i386 firewall of mine, although a bit rusty wrt hardware, is 
working wonderfully and is allowing us to fully exploit our Internet 
connections (1Mb/s).

Today it suddenly dropped to a bare few b/s. I checked the ISP line by 
attaching another machine in place of this and it could do full 1Mb/s, 
so this box was the problem.

After a simple reboot it started working as good as always.

Now the question is: in case this happens again, how do I find out 
what's wrong?
CPU usage was under 2% and so was swap usage... what else could I check?
What tools should I use?

  bye & Thanks
	av.