Ubuntu – Wi-Fi speed issue with 14.04 LTS

I have a strange issue with the Wi-fi speed in Ubuntu 14.04. After a restart of the computer my wlan speeds range at 20-30 Mbs, but after a while of usage (and/or idle) the speed drops to about 2-3 Mbs. After disabling and re-enabling Wi-Fi, the speed is back in the 20-30 Mbs range. I am measuring the speed with http://www.bredbandskollen.se/ which is known to give reliable assessments of network speeds.

I have tried some of the tips here: http://itsfoss.com/speed-up-slow-wifi-connection-ubuntu/ but it makes no difference. My wireless adapter is ASUS PCE-N15 WLESS PCI-E 802.11N 300MBPS. I get the following output when I run ifconfig, lspci and iwlist power.

Any ideas?

...~$ sudo ifconfig wlan0
wlan0     Link encap:Ethernet  HWaddr ac:22:0b:91:9d:d0  
          inet addr:192.168.1.197  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::ae22:bff:fe91:9dd0/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:218256 errors:0 dropped:1 overruns:0 frame:0
          TX packets:200633 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:149638799 (149.6 MB)  TX bytes:88264551 (88.2 MB)

...~$ lspci
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 0b)
03:00.0 Network controller: Realtek Semiconductor Co., Ltd. RTL8192CE PCIe Wireless Network Adapter (rev 01)

...~$ iwlist power
eth0      no power management information.

lo        no power management information.

wlan0     Current mode:off