No duplicate IPs, DHCP is consigned to the 200-240 range.
I'll diagnose more next occurrence.
Can't be IPTables as it is fine for a while with correct arp table entries, even if I delete them, a quick ping and the ARP exchange happens fine. Then suddenly 101 can't see 3 again.
To be honest, 101 is an old PI 2B and it's been annoying me for other reasons. Maybe I should just replace it with a PI3. I happen to have one lying around as I replaced it with an ESP8266.
On other thoughts, check this little exchange out that happens a lot, commands are run meer seconds apart:
paul@localhost ~ $ ssh pi@10.0.0.101
ssh: connect to host 10.0.0.101 port 22: No route to host
paul@localhost ~ $ ping 10.0.0.101
PING 10.0.0.101 (10.0.0.101) 56(84) bytes of data.
[b]64 bytes from 10.0.0.101: icmp_seq=1 ttl=64 time=1031 ms[/b]
64 bytes from 10.0.0.101: icmp_seq=2 ttl=64 time=29.1 ms
64 bytes from 10.0.0.101: icmp_seq=3 ttl=64 time=1.15 ms
^C
--- 10.0.0.101 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 1.158/353.988/1031.705/479.354 ms, pipe 2
paul@localhost ~ $ ssh pi@10.0.0.101
pi@10.0.0.101's password:
Frequently 101 will not answer to SSH. Even when I couldn't remember it's IP I scanned the whole LAN with NMap looking for SSH ports and it didn't find the PI. But if you ping it, it suddenly appears.
I'm thinking either 101 has dodgy networking of some kind or a dodgy USB Wifi dongle.
Another reason to replace it with the PI 3.