The first day i had the same problem with wifi signal. i tried all that about drivers etc. but the problem was there. Somewhere in a reply i read about changing the wifi card so i opened the back cover to check what wifi card i have. The wifi card has two wires. one black and a white one. The white was disconnected. when i press it and it connected on the antenna the signal was perfect. so friends check your antenna wire. i hope this will help.
sorry for my english.