Odd New Year problems with network accounts


situation is: 40 odd Minis on 10.11, MacOS Server running on 10.13 with 150 networked Home Folders on 2 external USB3 HDD drives.

Gigabyte fibre network with Cisco gigabyte switches

Before Xmas - all was well. Logins smooth and speedy, even with sixteen users logging in at the same time.

Now - network logins often very slow after a period of unuse - and I mean five minute login time. Progress wheel stalls for much of that time. After a logout, second login is quick. (10-20 secs). On a different client as well.

If you try to login on a second workstation (whilst the first is stalled) it seems to unclog everything.

Leave workstation for a couple of days and cycle repeats.

Server shows no rogue processes hogging CPU or Memory pressure

Can anyone shed any light on this? I'm stuck!