HELP! nVidia GForce4 MX440 - screen goes blank?!

I have an nVidia GeForce 4 MX440 8X 64M DDR AGP card. My motherboard has
on-board video which automatically disables when an AGP video card is
present.

This card works fine EXCEPT when I access the network (internet or
whatever). After a few seconds, the card will stop sending signal to the
monitor (the monitor's led goes from green to yellow as if the PC was
turned off, or the card went to sleep).

I checked for conflicts with the network card, but couldn't find any.
The video and network are using different IRQs, memory, etc. I also
upgraded the BIOS on my MB and that didn't help either.

Anyone had this problem or heard of it and know of a solution?