Has your neighbour suddenly started using 5ghz or is 5ghz still relatively free.
Has the router been moved at all?
Not forgetting that some higher 5GHz "sub-bands" like UNI-II/ext and UNI-III are constrained by DFS/TPC and therefore liable to co-lateral co-channel interference from various forms of military and civilian (weather) radar and as such you find both SH3/VMDG505 and SH2AC may well take some time in assessing local RF bands to ensure that priority traffic on frequency is detected and WiFi Rx/Tx reduced or even negated depending on Auto/Manual options selected.
Unless you/we have full 5GHz Spectrum Analyser and also correlate our GPS location/distance from military,airport or shipborne horizon radar then it's entirely possible that our relative 5GHz bands may vary respectively between users. Due (I believe) to resource issues on SH3 that have existed for last 17months since it's trials/introduction then some of the upper 5GHz DFS/TPC bands can take nearly 10mins after a reboot/change compared to a minute with SH2AC.
Point is that 2.4GHz band is complicated enough even with 22MHz overlapped channel bandwidths but 5GHZ non overlapped channels are still constrained in that unlicensed Band A (ch 32-64 is limited to 200mw eirp and and only 32-48 is not liable to DFS/TPC) while Band B (100-140 is 1000mW but all limited to DFS/TPC). Band C (4000mw) being out of scope of this residential forum.
So our mickey mouse (non pro but FREE) Wifi Analysers invariably mean if we don't see any co-channel WiFi usuage then we immediately (naively) assume that the performance problem is entirely due to VM Router hardware/firmware! "Oh duhhhhh" or at least "Hmmmm...." LOL
Regards Tony "Life is a Binary Inspired Turing Computed Hologram"(don't PM or @Mention me - in case ignoring you offends)