Network slows down when run on 100 BT

There are 30+ systems on the Network running win98, and we have a NT server. When on 10BT the network runs with no issues.
I changed the 10BT hub to a 10/100 BT 3Com 3300 TM Switch, from then on I have been getting slow connections to the server, especially when using a Database from the server. All the NIC cards are 10/100 3C905x and few Intel 8255xx. Even when I forced the NIC’s to 100BT & run the network I get very slow responses. I also tried to force the switch to 100BT and in full Duplex Mode but still no luck.
If anybody knows how I can come around this problem could they give me a few pointers please.
Darrell…