I have a Linksys E4200 dual-band router. In fact, I replaced it before coming to the Intel forum to make sure that it wasn't router hardware causing the following issue.
My 2 GHZ WAP gets 65Mbp/s to its connected devices that do not have the Intel chipsets. However, the 5.4 GHZ WAP gets 48 to 54 Mbp/s in a fluctuating manner. I only have two Intel chips and the two devices using them are what connect to the 5.4 GHZ WAP.
The two laptops operate with Windows 7 and Ubuntu 11.10. Both operating systems have the same results for connection speeds. The WiFi cards are Intel Centrino Advanced-N 6200 and 6230. Both fluctuate at 48 - 54Mb/s on the the 5.4GHZ channel.
If I put the 5.4GHZ on Wireless-N only, the laptops cannot connect; it has to be on MIXED (A & N). I have tried AUTO detection for mhz and channels as well as 40mhz with channels 149 and 161 with no difference.
Currently, I am operating both WAPs on different SSIDs to make sure my results are separate.
Security for both WAPs is WPA2-Personal with a 10 digit sequence.