Had similar issues with 2 different WZ's.
One had a range on WiFi of less than a couple of feet
The other, similar to you, WiFi and BT worked OK for a couple of weeks then WiFi stopped working, BT continued to work.
Swapped SD cards, used SD cards from a working Pi ZW, updated kernel etc to no avail
Returned to vendor for replacement, can only assume a dodgy wireless chip so no software fix will work