Earlier this evening, my Imp started randomly going offline, and sometimes coming back online. Really infuriating! Hardware seemed OK, so I started doing some logging, and it turns out the RSSI was at -89, which is a bit too low! So I moved the picture frame that my wife moved earlier in the evening to make way for the Christmas tree, and lo and behold, RSSI of -81 and everything running smoothly.
Looks like my garage needs its own access point!