IMacs on Appletalk Network – speed

We have been replacing our LCIII’s, 5200 series, etc. with IMacs. It seems that as we replace more older systems with the IMacs, our networks are experiencing considerable speed drops, and much more frequent lock-ups.
It has become very common forour connection to AppleTalk Servers to drop, however the IP connections to the internet and email remain. Are there any conflicts between the new IMac’s and the older systems. Most of the buildings still are using 6100 series boxes as servers. Whenwe pull the IMacs off the network and replace the 5200’s etc., the problems seem to disappear, or are lessened.