Try to set up the router in 40 MHz (wide channel) and the Intel® wireless adapter in Auto. It is recommended to check that the WMM option is enabled on the QOS settings as previously suggested in the thread below. The suggested security option is WPA2-AES instead of TKIP options.
You may also update both wireless driver and router’s firmware.
Thanks for your advice. I will try and advise outcome in due course.
I've had a similar connection problem with the Intel N-6235 on a Netgear router. I had very low download speed despite 802.11 n support on both adaptor and router. This worked for me (after countless different settings): set your router to use the 40mhz band and enable WMM support (in both the router and the N6235). That's it!