Dude! Your fone is atleast 4 months old. Im facing the same problem on my 6630 from DAY ONE! Its damn irritating. This is my 8th Nokia phone, and no other phone before gave me such trouble.
Its sad to say and painful to listen to, but i must tell u that my friend also faced the same problem of flickering screen and horrizontal lines on his 6630 screen, and he had to get it replaced from Nokia. His screen was changed!
I hope we dont face the same thing with our phones...but lets be prepared to expect the worst and hope for the best!