Wireless problem

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.

In Ubuntu I've read that it can be connected to some kind of network manager that scans for better channels/ap to connect to.

Is this something that is available in LE also? Is it possible to disable this scanning?

Any other suggestions I could try is also appreciated.

Quote from Sholander

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.

But will that stop the Pi3 from trying to find another channel? Or does the AP "tell" the Pi3 that this is the only channel you can use, don't try to find another if I set it manually?

I have now tried and tested several channels and still have the same problem. And it is so periodical so I don't think it is interference.

Using another wireless computer when copying a file doesn't show this behavior. Is there anything else I could try?

This is a example of the speed. Even though it doesn't look like it, the valleys is often close to 0Kb/s. And when that happens LiveTV dies.

To me it feels so regular that it must be something else than a interference.