UBUNTU 16.04 LTS: WLAN-Bandwidth unsteady, keeps decreasing, keeps deconnecting

I really need your help!

Recently, I switched from Win7 to Ubuntu 16.04 LTS in order to avoid WinX.

So far, everything worked out fine. Until I discovered, that my WLAN-bandwidth keeps on altering - in most cases: decreasing - after irregular intervals.

• I usually start with a rate of >100Mbit/s
  (I'm using my routers monitor-website to check)
• After a short time, I'm down to ~54Mbit/s, usually, I won't notice, because browsing and streaming is still possible.
• Adding some more minutes - regardless whether I put load on the connection or not - the rate drops to 5MBit/s or even 1Mbit/s!
• Sometimes WLAN will even disconnect and I have to restart the NetworkManager-Service to regain access.

When connecting my smartphone to the wlan, it keeps a steady rate of 54Mbit/s, which is as expected.
Back in the dark old days of Win7, a behavior such as mentioned above, was never noticed.

Well, despite of being new to Ubuntu, I'm a rather confident son of a bitch. I already tried various "fixes", such as disabling the N-standard, compiling and loading a new driver,... but to no avail.

Wireless Info Script

Some additional Info:
GUFW ist installed but not active and maybe configured in a wrong way. (Yes, I am new to a non-Windows-System)

Please help me with this mess, I don't want to go back to Windows...