I have a issue with wireless performance on my Pi3. I thought that it was a problem with the integrated wireless and added a USB adapter. But I still have the same problem.
Transferring a file normally gives about 4-5MB/s, but very periodically it goes down to a few Kb or even zero for a second or two, enough to mess up LiveTV or any other streaming.
Install a WiFi Scanner app on your mobile, and scan WiFi channels placing the mobile near your Pi. When you find a free channel (or one with least interference from neighbors) set that channel manually in your Router/AP.