I think the problem is the 802.11 mode. Mine is also set to 300Mbps and my laptop with a built in N wireless card loses sync and auto reconnects at specific intervals (haven't timed them but it could well be 15 mins).
Set your wireless mode to 145Mbps and try channels 7-9. Also use the tool below to see if there are any local networks interfering with yours:
http://www.metageek.net/products/inssider/
Failing this, try a spare router if you have one to see if the adapter loses signal. It may well be a faulty adapter
