MS Explorer Loosing Navigation To Workstations?

I recently upgraded a client's computing site installation. 4 Desktops, software (XP/MS Office 2003 to W7/MS Office 2010) and one MFD. To save some time and make the transition smoother (and attempt to iron out any unforseeables) I set the 4 new workstations up at home - at cat5 hardwired through a TP Link Router under a Workgroup, with each workstation named WORKSTATION01, 02,03 and 04, with file sharing via the workgroup, appropriate advanced sharing center settings and routing via DHCP (dynamic IPs). The network worked flawlessly at home for two weeks, whilst I installed the inevitable updates and made sure when I installed at the site, everything was as up to date as possible and any issues ironed out.However, the client started loosing the ability to navigate to another WORKSTATION via MS Explorer typically, it would happen in the afternoon. Some days, it wouldn't happen at all.BUT! WORKSTATION01 never looses Workgroup visibility in MS Explorer only the other 3 do - and not all at one time always.I've compared all settings on WORKSTATION01 with the other (offending) 3 and they are the same.

All work stations peer at the same level - there is no (higher) server as such. Although WORKSTATION01 has the clients accounting software data file on it and important documents are stored under a SERVER folder which is resident on WORKSTATION01's hard drive.The "visibilities" are always fine when they boot the machines up in the morning. If a device looses the MS Explorer visibility to the Workgroup (under Network) - sometimes rebooting the device restores it, some times it doesn't.They don't loose the MS Explorer visibility if they leave a session(dialogue) permanently open - but they shouldn't have to to that.I've set up a desktop folder with inter-workstation short cuts as a work around and they always work.
Other Considerations:

1) SOFTWARE - running Windows Defender with Firewall On and Avast.

2) FIREWALLS ON/OFF - I've read a lot of threads - the jury seems out on this - some advocate turniong them off, others fervently announce this should NEVER be done - use the right settings!

3) DHCP vs STATIC IPs - again, plently of dispute on this as to whether DHCP or static IPs should be used for device routing.