Having exactly the same issue.
Wifi on brand new XZ drops out all the time. Most noticeable streaming music from internal SD card to Sonos system. Same SD can in my old Z3 works perfectly, same again in a loan XA I've got for testing. Raised a support case with Sony who said I need to change the handset so Vodafone replaced with another new XZ, which has exactly the same wi-fi issue. It's not the Wi-Fi network I'm connecting to as it's exactly the same on different networks (home, work, other friends that also have Sonos). All tested networks work perfectly on the Z3, XA and Apple devices. Sony support asked me to send handset for repair, same handset returned to me noting that there was no fault found and the handset complies with "industry specification". I don't even believe they've tested anything other than connecting and authenticating to a wireless network.
Only common part here is the XZ. It's obviously a design flaw or hardware issue in this model, but Sony Xperia support just keep sending me the same generic text response about posting is back if I think there is still an issue. Why can Sony not test this on their own XZ and just acknowledge what is an obvious issue with this model and provide a timeline for resolution.
