HI,
did you try switching plain-mode on/off? My UX with AC 8265 on 802.11ac network sometimes enters limited 30Mb/s speedtest speeds. Switching to plain mode for few seconds then turning it off returns to max available internet provider speed. Don't know what causes it. 30Mb/s is enough for daily tasks so I rarely notice it.
Did you try installing drivers manually? Does driver version changes between bad and good? If not, then certainly it is not driver issue. If indeed driver version changes, then perhaps update driver from device manager selecting your own driver. Feeding Windows with right drivers usually doesn't fail until major Windows update. AC 7265 never caused problems to me on Dell laptop. Are you sure it is not AP / router fault? WiFi cards sometimes die as well.
Didn't your WiFi channels change? It is not clear which band are you using, both 2.4 and 5GHz are bad, or just one of them. At 5GHz you may face some DFS issue, channel may change, signal power from router may drop. Did you try cycling your APs/routers on/off? If you think it's SW problem, perhaps try creating Ubuntu live USB using Rufus. Boot from USB and see how your WiFi behaves, perhaps disabling WiFi several times and checking signal levels and connectivity. It would help checking your WiFi card.
Edward