Had this problem with a Dell Vostro 3700 and replaced the WiFi card with an Intel wifi card 6300 833AN-HMW. It connected instantly and has not dropped WiFi in over 2 hours.
Until I received the new Intel card I used a USB 802.11n plugin WiFi that is about an inch long (Ebay: 802.11 Dongle RTL8188) that did not drop the WiFi for over a week.
Spent two months trying all suggestions on various forums.
Bought a $20 USB Wifi adapter and shut down the internal one. No issues since then. Hence it must be a hardware issue as I have completely updated and reset and reconfigured the internal adapter over many many weeks.