Onboard ethernet adapter very slow compared to network card - why?

Hi there,

I recently acquired a PC with a PC Chips M863 motherboard with an onboard ethernet adapter - a SIS 900 to be precise (10/100 Mbps). However when I connect to the internet using it it's very slow. Even accessing my router's setup (192.168.0.1) is slow.

If I connect to the internet using my Lite-On PCI card from my old computer (which is 100 Mbps) it's fast again. But when I plug it into the onboard... slooooooow.

This wouldn't be a problem but my mobo has 2 PCI slots - one for my SATA card, one for the network card. I have recently acquired a graphics card that is so big you cannot use the PCI slot next to the AGP slot.

I need my SATA card so I am hoping to get the onboard ethernet to actually work... I've updated the drivers using the most current version from the SIS site (which I thought worked for a few joyous seconds until I realised Firefox was loading the pages from the cache! )... can anybody suggest anything else?

Thanks a lot.