Before 3-4 days surfing the Internet began to be very slow.
Very frequently the following error message is displayed:
"HTTP Error
Status : 504 Gateway Time-Out
Description : Unable to connect to origin Web server."

I checked with the ISP and from their point of view it looks OK (Upload
1.5 Mb \ Download 270 Kb).

Our system is based on SBS 6.5 sp1, with 3 servers (BM3.8, GW-DNS\DHCP
and WEB).
We have only 3 WS (Win 98 & Win 2000).

What and how shall I check?