Have you checked your SCART leads are fully connected? I'm wondering whether you are actually using composite all the time. To check this, connect the TV direct to the STB, and toggle the STB's setting between composite and RGB. You should see the picture "change" (usually you get sharper edges but less vivid colours on RGB). If not, you are only getting composite and should look for new leads.
I was browsing the Panny website and found this...
http://www.support.panasonic.co.uk/s...aflet3-new.pdf
Does it help? (Probably not, but I thought it was interesting that they considered it worthwhile producing a supplement. It suggests you're not the first to have this problem. You could try asking them).