No internet brings intranet to a crawl
 

Situation:
adsl modem alcatel home touch
linksys 8-port router/switch
fixed ip's

1 server / intranet
6 client pc's
no dhcp

Problem:
If the i-net adsl-connection drops, the intranet works extremly slow. Shared files, http-requests, ... whatever, ... all requests from a client pc to the server result in very long wait times or even time-outs.
If the inet connection is restored, everything works back at blazing speeds.

I've searched extensively to a possible solution for this problem, but i cannot even find the key/pointer to where the problem(s) may be.