Permanently poor WiFi on my Macbook Air

I have a MacBook Air (late-2010 model). Recently, whist on holiday in Asia, my laptop malfunctioned and a trusted friend took it to the local authorized service provider. They diagnosed a hardware problem and exchanged the power supply component. The laptop worked again as before. Great.

However, since then I have been encountering permanent, persistent WIFI connectivity issues. The WIFI signal may be indicated as strong, but when I attempt to connect to a network it either takes far too much time trying, it may drop the connection attempt altogether, and if it does connects, it often fails to find the router.

The WIFI diagnostics tool suggested that the country code diverges from that of the router (that should not be a show-stopper though). It also complains about a noisy WIFI environment. However even if I sit next to the router, the problems persist. If it does connect and succeeds to find the router, my internet connection is ridiculously slow. Using Alt + click on the WIFI icon shows that the signal strength as well as the noise level are at - 89 dBm.

What could the problem be?

I think it's a hardware issue as I have already done a disk repair and safe / restore boot. It was also suggested by some user posts that I read here of reports where the antenna was not correctly hooked into to the wifi chip.

Any suggestions?