Just purchased this router today. First thing i did was upgrade to the latest firmware and cleared nvram for the heck of it. My Vaio laptop with an Intel-6235 would only connect if i changed the 5 ghz setting to 20 mhz. If the setting was at 20/40 or 40 mhz it would not connect at all on 5 ghz. It would connect to 2.4 ghz with no issue. The highest link speed i could achieve was 144 mbps on 5 ghz. I then installed Merlin 372.31 cleared nvram and now i can connect with the setting at 20/40 and the link speed i expected of 300 mbps. At least for me the new firmware is not the answer.
Make sure you use an up-to-date driver for your Intel card. The latest is 16.1, but it was also working fine for me with 15.8 and a 6230.