Unusual JVC TV problem - fluctuates between colour and b&

[yb]

I have a problem with my JVC TV when connected to a sega megadrive (via the video RF port). The image starts as black and white, then the colour level increases for 10 seconds before the screen refreshes as black and white, and the process repeats indefinitely. The TV works fine otherwise, and the megadrive works fine with other TVs. I've tried playing around with the screen settings and service menu without success. For example I tried turning the colour level down but this doesn't stop the refreshing. Is this likely to be a hardware problem, or are there any options I could tweak to solve the problem?

Thanks in advance.

[yb]

Sorted now - TV needed fine tuning (though why the default was that behavior I have no idea)