I have a problem with my 37LG30; when I first turn

I have a problem with my 37LG30; when I first turn the TV on, I get sound but no picture. The picture then flickers on & off, with horizontal lines appearing. This has been a problem since new but is getting progressively worse, now taking 20 minutes or so for the picture to come on fully. I did initially think this was a problem with either the AV sockets or the SCART lead however I don't think that this is the case as it has now started to happen through the HDMI socket (although the problem isn't as bad). Any ideas on what is causng this and how it can be fixed?