Actually this problem appeared few months after I bought the MBP, but then I did the power reset (took battery out and hold power button for 10 secs), and the problem disappeared until last week when I did a new installation of SL. So then I did the same thing, it worked fine for about 2-3 days.. then appeared again.. then I did the PRAM reset.. worked for 3-4 days.. then.. just before it happened again! I just did the PRAM reset the second time.. Wonder is this a hardware problem? Should I send my MBP for servicing? Thanks!