Sometimes can't access device on local network

[Colin Law]

I have an odd situation that occurs occasionally.  I have a conventional
wired local network (192.168.1.nnn) with in addition some devices connected
via wifi. The main router (192.168.1.1) is also the wifi hub.

Consider three of these machines. A, B and C.  A and B are wired and C is
wifi, all with fixed IP addresses.

Normally all is well but occasionally I am not able to access C from A.
When I ping it I get
$ ping 192.168.1.85
PING 192.168.1.85 (192.168.1.85) 56(84) bytes of data.
>From 192.168.1.92 icmp_seq=1 Destination Host Unreachable

92 is the device I am running on (A).

However I am able to access B from A and also able to access C from B,
which is particularly odd. So, even though I cannot ssh into C from A, I
can ssh into B from A and from there ssh into C.

Can anyone suggest how I might go about debugging this?
I don't want to reboot things at the moment to see what fixes it in case
anyone suggests something I should look at, so I don't want to stop it
failing yet. It does not happen very often.

I don't have any other network problems that I am aware of.

Colin
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