Sony Vaio weak WiFi signal

I've got Sony Vaio SVF1521A1EW (white); Windows 8. Recently I've been having problems with the WiFi connection. At the bottom right corner the icon would show only one bar signal (out of 5) or no bars (no connection) at all. I don't have this problem when connecting on my other devices (laptops and smart phones). I have tried all of the troubleshooting things and none of them worked. Today I decided to reset the laptop to fabric settings - same problem.
The network adapter in use is BCM43142 Wireless Network Adapter -- Windows says it is working properly as are the rest of the drivers.

Please, tell me how to fix it. Thanks.