DELL XPS 13 9350: no WiFi, when Bluetooth is turned on

Good evening,

since i bought my DELL XPS 13 9350 im facing several Wireless problems. The build in DW 1820A is really weak. But the biggest Problem is, that i just cant establish moderate Internet-Speed, when Bluetooth is turned on. Everytime, when BT is turned on, the WiFi Speed just decreases to a Level, at which not even the Google page can be opened. DELL Germany send me a new DW 1820A, but the same Problem is occuring with the new Card.

However, now DELL Germany send me an Intel 18260. The WiFi-Speed increased compared to the DW 1820A. Furthermore, when BT is turned on, the WiFi Speed is stable. However, the BT has another Problem: While BT devices are shown as connected with the Notebook, they dont work really well. Headset dont stream Audio properly. After 30 seconds the Audio just is not streamed anymore, while the device is still connected.

If someone knows any solution, i would be very grateful. 

The newest Drivers are installed for the Intel 18260 and (i think) all Drivers and apps for the old DW 1820A are deinstalled (Network and BT-Drivers, Wissdom? Bluetooth app, DW WLAN Card app).