I have MacPro1.1 with 7gb or ram using OS10.4.11 and Safari 4.1.3

After multiple large downloads such as 15gb or more Safari Crashes. When I relaunch Safari it takes a while-much longer than usual and it sounds like the Hard drive is writing or Accessing like crazy while safari is loading. This may last 2 to 3 minutes.

It's as if Safari gets "full" and can't take anymore so it crashes. Then when I relaunch and it takes a few minutes to load up while listening to the hd as if it were purging itself empty and when it is Safari is back up.

At this point it seems prone to crashing whle surfing and downloading simultaneosly. It seems restarting the Mac "clears it out" the best.

Any ideas what this is and how to stop it?