Strange network (wifi) issue with SMP

I have been running 2 WDTV Live (gen2) boxes for a few years with no issues at all. 1 machine is in my living room, connected to a Netgear Router by Ethernet. The other is upstairs and connects via wifi using a DLink-140 usb wifi adapter.

Never had any problems at all.

But, a very strange thing is happening since I connected the SMP in place of the unit downstairs. When the new SMP player is used all works fine. But once I power it off and go upstairs the older WDTV Live is unable to see the network shares. It seems to pick them up (sometimes) but will not connect - and they they disappear from the list. Strangely, my mobile phone also loses the wifi connection at the same time. If I reboot the router then the old WDTV and my phone can then see the network and all is fine - until the new SMP unit is turned on. Then it happens again.

The new SMP is running the latest firwmware, as are the WDTV Live (old version) and my router.

I have tried a second new SMP and the same happens.

I am really confused as to what is happening. Does anyone have any ideas?

Did you remember to change the name on one of the boxes?  They will both have the same name by default, which will cause all sorts of issues.

Yes. Did that - One is WDTVLIVESMP, the other is WDTVLIVEUP.

Different IP addresses?

Yes. I have tried iving them both static IP addresses, and also letting them get assigned an IP by the router. Same happens.