Small network running slow

I have a small network just running a workgroup and sharing files from one main pc.  It has recently begun to run slowly for no apparent reason.  

I suspected the 8 port switch was causing problems and have replaced it.  It seemed to run faster, but only for a couple of days then back to slow again.

I have replaced what I thought was a bad cat5e cable, again it seemed to speed up then two days later slow again.

The speed issue is mainly showing up when using Quicken and especially from a remote or client pc within the network.  From the Server/Workstation the program appears to run normally and the speeds from the client pc's have been good until recently.  Nothing has changed enviormentally and the software has not been updated, memory and processor is not maxed out on Server or client.

Looking for any answers to why it is slowing down and would like to know if there is any free/cheap software that could be used to identify the bottleneck.

Thanks in advance