I have a new Samsung ATIV 940X laptop running Windows 8.1 with the Intel 7260 chip & have been having the same problems with it as others have been reporting in this forum. I have tried all the things various people have mentioned (uninstalled all the old drivers, installed the latest Intel driver (17.0.5.8), disabled U-APSD, disabled HT mode, changed all the power settings, etc.) but still experience much slower internet speeds than I should be getting.
I have a 4-year old Dell laptop sitting right beside my Samsung and it gets about 70 Mbps download from my router while the Samsung gets about 23 Mbps from the same router (about 6 feet away from both).
So, since I had an old Trendnet USB wireless adapter (TEW-664UB) sitting around that I wasn't using I decided to install it in the Samung and disable the Intel 7260. Yep, you guessed it - the Samsung now gets about 80 Mbps download speeds with no disconnects or other problems. The Trendnet is using the default Windows driver (5.1.2.0).
The router that both computers are communicating with is a Comcast supplied Arris TG862 Gateway (combination cable modem & router). The router only puts out 2.4 Ghz Wi-Fi, no 5 Ghz.
Seems to me that this clearly indicates some kind of problem with either the Intel chip or the driver for it.