I'm experiencing these issues on a Lenovo t431s 'premium' flagship laptop with Ubuntu 12.04 and the stock iwlwifi kernel drivers. After many days of experimentation the only solution I've stumbled on was setting the router to N only mode at 5GHZ which alleviates the drop out issues. The signal strength is dismal, though. Sitting next to my router I'm at 50% signal strength.
This works for me at home but taking this elsewhere I've had to resort to a USB adapter. I would be happy to pay for a replacement card from a different vendor except for Lenovo's bios hardware white list and the fact this bundled with bluetooth.