Ubuntu – bad wifi connection with Lenovo X240 under Ubuntu 14.04 — intel wireless 7260

On my Lenovo X240 I have installed Ubuntu 14.04. Everything, including the LAN connection, works very smoothly except from the WiFi.

The computer connects to a WiFi hotspot but after a few bytes transmitted the connection breaks down completely.

EDIT: –> If the hotspot is less crowded, i.e. if I switch off my other clients, then the connection works more or less stable but still with occasional troubles. <–

The hotspot is OK, since other devices do not experience any troubles.

Any ideas? I am not too deep in Ubuntu, please give me advice if you need more information on the hardware.

This is what iwconfig gives me:

iland@note121:~$ iwconfig
eth0      no wireless extensions.

lo        no wireless extensions.

wlan0     IEEE 802.11abgn  ESSID:"thrasher(at)hajod.de"  
          Mode:Managed  Frequency:2.412 GHz  Access Point: 7C:4F:B5:C8:D2:C4   
          Bit Rate=54 Mb/s   Tx-Power=16 dBm   
          Retry  long limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=70/70  Signal level=-35 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:6   Missed beacon:0

And this the hardware configuration from sudo lshw -c network:

 *-network               
       description: Ethernet interface
       product: Ethernet Connection I218-LM
       vendor: Intel Corporation
       physical id: 19
       bus info: pci@0000:00:19.0
       logical name: eth0
       version: 04
       serial: 28:d2:44:40:96:64
       size: 100Mbit/s
       capacity: 1Gbit/s
       width: 32 bits
       clock: 33MHz
       capabilities: pm msi bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt-fd autonegotiation
       configuration: autonegotiation=on broadcast=yes driver=e1000e driverversion=2.3.2-k duplex=full firmware=0.6-3 ip=192.168.1.11 latency=0 link=yes multicast=yes port=twisted pair speed=100Mbit/s
       resources: irq:61 memory:f0600000-f061ffff memory:f063e000-f063efff ioport:3080(size=32)
  *-network
       description: Wireless interface
       product: Wireless 7260
       vendor: Intel Corporation
       physical id: 0
       bus info: pci@0000:03:00.0
       logical name: wlan0
       version: 83
       serial: 5c:51:4f:58:a5:c5
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list ethernet physical wireless
       configuration: broadcast=yes driver=iwlwifi driverversion=3.13.0-24-generic firmware=22.24.8.0 ip=192.168.1.10 latency=0 link=yes multicast=yes wireless=IEEE 802.11abgn
       resources: irq:66 memory:f0400000-f0401fff