Question : networking nightmare; can it be fixed

I have a dental clinic client (inherited this) who has 23 computers, about 7 of them used for pateint exams. I putin a brand new server, which was as Identical as possible to the previous server, except for the server name was server1, old one was server. I had to demote the workstations and put them in a workgroup and them promote them into the new domain. Most of them went fine. Got everythingup and running. Now I have several problems, some of which appear to be long term intermittent.
Network will run fine for a while then suddenly the whole network will crash
during the crash I lose almost all connectivity between the server, PCs and the cable modem (SMC from Comcast).
during the crash I can plug my notebook directly into the SMC nodem and not be able to ping it
same thing with the server, using regular cable and cross-over
unplugged the switch connections one by one and when I unplugged port 19 the network came alive; as soon as I plugged in 19, with 2 minutes it crashed again.
Also have three front desk PCs which go through an older SMC ez-switch which was left behind and when I plug my notebook into that switch I cannot ping server or modem; plug it directlyi into the jack that the switch plugs into and I can ping
Got the netwokr running again last night at about 11:30 and this morning it appears to be crashed again - both port 19 and the suspect switch are disconnected fromthe network.
Highest priority is to get the patient exam pCs connected to the software on the server, second priority is front desk people, third and probably most important priority long term is to get the network stable
I have already spent over 40 hours this weekdn trouble shooting it and need some serious help.

Answer : networking nightmare; can it be fixed

Also are there any errors on your server in the event log and have you tried setting up another nic on that server and disable the one your using now?