Win98 NBT upload problems

Hi everyone

Today several Win98 clients uploading files to W2K server is very extra slow. They are running NetBios over TCP/IP. But download file from server, speed is normal

I tried install IPX/SPX to replace the TCP/IP protocol. The upload and download speed are ok. But I don't want IPX/SPX as network protocol. I tried replace the network card with other brand name, it is running at normal upload speed also, but everyone will think that only stupid would replace all the network cards

What I did beforehand is that I installed Windows W2K DNS server, dont know whether it is the case, but now that DNS server have been uninstalled. Win98 clients uploading problem insists

Anyone can help? Thanks!