I have a Sony KDL40X350 less than 3 years old purchased in Bath from the Sony Centre.
I experienced the same problem as you have after about 18 months and Sony changed the screen. Now coming up to 3 years the problem came back again and it has gone back to Sony for (guess what!) another new screen.
I have talked at length with the Sony people as I cannot believe the problem can stem from the screen as I will now be on my 3rd. I think the problem is coming from the input voltages with perhaps too much heat being generated. Sony have said if it happens again (I think it will) they will give me a new set.
I wouldn't have bought a Sony if I had imagined it would be so much trouble as why not spend a fraction of the cost and get a (reliable) LG. I thank my lucky stars that when i bought the set it came with the special offer of a 5 year warranty; don't buy a Sony set without this essential piece of paper!
I suspect my expensive Sony will end up in the skip when my warranty runs out and I will get an LG
Good luck