I found a way to fix this Safari 5 problem with my Billion 7300A modem.
The problem seems to be the way Safari 5 handles DNS pre-fetching which seems to be over loading many modems.
First download Googles' "Namebench" application from here and run it (Make sure "Include global DNS providers" and "Include best available regional DNS services" are both checked):
http://code.google.com/p/namebench/downloads/list
Now see what is the fastest DNS Server available to you(FASTEST,not fastest + nearest from top right hand corner).
If your ISP supplies the fastest DNS Server (like mine does) go to your ISP account and look for something like "adsl settings".There you should find the primary DNS Server (number should match DNS number from Namebench) as well as a secondary DNS Server number.Write both of these down.
If you got something like Google DNS,Ultra DNS or Open DNS as the fastest,go to their websites to find their secondary DNS Server number.
This is how I did it on the Billion 7300A with a MacBook Pro running Snow Leopard 10.6.7.First go to system preferences > Network > Advanced > DNS >Now click the "+" button and enter the number of your Primary DNS Server.Click the "+" button again and enter the number for your Secondary DNS Server.Click "OK" and then click "Apply".
Now bring up your modems' settings (usually done with a browser like Safari or Firefox).You can find the number to put into the address bar,in the manual that came with your modem (in the Billions' case it's: http://192.168.1.254/index.htm)
Now in my case it went like this.Modem settings > Quick Start >Now go to section headed "DNS" and uncheck "Obtain DNS automatically" if it's checked.
Now there should be nothing in the boxes marked "Primary DNS" and "Secondary DNS",but if there are numbers there,erase them.Now click "Apply" and click "Save Config".
You may need to restart your browser but basically,that's it.You're done.Safari 5 should be screaming along.