How do I solve my Sagem HD D45 picture issues?

I have a Sagem HD D45 and am experiencing some major picture problems. The TV comes on fine but after about 20-25 mins the picture starts to flicker the colours until eventually everything that the RGB colours get mixed up (red becomes green, green becomes blue and blue becomes red). Eventually the colours do settle back to normal (anything between 10 mins and an hour) but the picture ends up looking 'solarised' with no real definition. I know the lamps ok as I replaced this about 5 months ago. Can anyone please help with this???? I have contacted Sagem but they were their usual vague selves and just said it was a 'mainboard' fault - isn't everything with them!