Internet connection slows after inaction

[boutland]

Running KNOPPIX 4.0 on oldish AMD K7. When system is booted up all is well with the internet connection (using a D-Link ADSL router, DHCP).
After a period of inactivity (not sure how long) the internet becomes more or less unusable, with huge delays in getting data.
Reboot and all is well again.
Any help with this would be appreciated.

[boutland]

Sorry, forgot to mention that Knoppix has been installed onto the HD. The ethernet card is a Realtek 8139.
John