I'm sorry to hear that you're having this issue.
It's difficult to pinpoint a diagnosis, although one of the symptoms may point to a power supply issue. As far as the dimming, I'm not sure what causes that, and that would need to be inspected by a technician. It's likely outside the warranty, but you can call 800-SAMSUNG to set up a service appointment.
I hope you can help. I bought my Samsung LNT 4669 Jan 19, 2008.
I am very sad to say that it is now having trouble starting up. When I turn it on it clicks and the red light blinks on/off. It now takes 2 min to start up and now the picture dims at times. One or two times the picture has gone out completely and the tv displays a blue trying to find signal alert. Is this a mother board problem or a power supply problem. Is this still covered by warranty?
Thank you for your help