Network slow or not working

We have a small business with 17 Macs (iBooks and iMacs) on the network. We have ethernet and airport. The Macs run either OS 10.5 or 10.6 depending on their configuration. (At this point we cannot upgrade some of the Power PC's to Intel so we are stuck with 10.5). We have two Linksys switches. We use an iMac running 10.6 as our server. We have a dsl line. Lately there are problems with file sharing on the LAN (cannot access the other computer) and then sometimes some computers cannot get on the internet or it runs like molasses. Most of the computers use ethernet although they have airport turned on also. The LAN problem seems to eliminate the DSL line as the culprit. Sometime restarting the server works, sometimes not.

Are there any suggestions as to where to look for the problem, e.g., switches, ethernet cables, airport, dsl, server, etc.

Thank you,
Linda