I very recently bought a T440s. After using it for a couple of days, I installed Win8.1. Immediately, I began to have the same problem as Kristianko and many other Dual Band N7260 have. After many days of trying many things, I went and reinstalled Win8. Haven't had a problem after that.
I have my doubts if the problem lies in the N7260 or with Win8.1. The reason I say this, is that I bought a USB wireless adapter, and the same problem I had with the N7260 I had with the USB adapter.