I continue to have problems with the wifi on my Surface Pro 3. Since the SP3 will never choose the fastest possible speed available on my Airport Extreme router I created unique SSIDs for the 2.4GHz and 5GHz bands. My problem is if I'm connected to the 5GHz band my connection will be stable for a few minutes then slowly start degrading until the SP3 completely drops the connection. The only way to get back online is to toggle airplane mode or select the 2.4GHz band manually and connect to it. I can reproduce this issue with ease. This is the problem with not having a wifi driver that can dynamically chose the best possible connection. Having to reconfigure your router then manually connect to or disconnect from separate SSIDs is a massive hassle and IMO unacceptable. I'm losing love for the product pretty quickly right about now.
I've written up a one size fits all post at http://digitalmediaphile.com/index.php/2014/09/16/change-config-or-spend-more-money-to-fix-surface-pro-3-slow-wifi/ take a look at the suggestions about channels and settings.
I read the entire article and it's very solid. Really well done but I've already tried everything mentioned. I have separate SSIDs, I manually chose different channels but if the Surface Pro 3 isn't in direct line of sight from my router the 5GHz connect is not stable. If I connect to the 2.4GHz band the connect is stable but slow. If I connect my bluetooth headphones to the SP3 while using the 2.4GHz band the connection is extremely slow! I own a Sony VAIO Pro 3 and a Macbook Pro, both with 802.11 AC capable wireless modules, and only the Surface Pro 3 is giving me these problems. The other laptops connect and stay connected for the duration of the time they're used. They also don't cap the connection speed at 144Mbps or below like the SP3 if I connect them to the 2.4GHz band.