Slight flicker from Philips TV

I've got a Philips 24PW6006 that is 3 yrs old. I've just noticed that the picture flickers in intensity slightly. It seems ok for the first few minutes after switching it on, but then it starts. If I switch it to an input where nothing is connected (eg AV) instead of getting a steady black screen, you can see it flashing, ie it gets lighter then goes dark again. The intensity of the flicker varies as does the frequency but it's typically around 2-4 times per second.
Any ideas what might cause it? I've checked obvious things like loose connections.