Thanks. I've tried two different wifi networks, with the same results. Low-level flooding the Unihacker with pings fixes it, bizarrely. All my other wifi devices don't have this issue.

I tried a USB ethernet adaptor and the connection is rock solid.

Unfortunately, it seems the wifi driver for the Unihacker is compiled into the kernel so I can't unload it and recompile an alternative. Perhaps there's a way, but this is beyond my knowledge.

So, the issue goes away if wifi traffic to/from the device is higher, ie fast pings (ping -i 0.1 -s 128 <IP>), or a lot of SSH traffic (htop -d 10).

I guess it's some kind of power save thing or something similar. Pinging the device with large packets is annoying, but it's a workaround.