is it hooked up to a VCR or some sort of external signal other than your aerial (PS2, DVD etc)
hows the picture then...if you still get the same problem its either a fault at the input of signal or the tube....is there a channel where the screen is all blue, or all black (like the AV channel)...if u still get the same problem then then i guess its definately something internal, possibly the tube or a related component (like the chip resposible for that area of the screen)
(btw im no TV expert and am essentially trying to figure this out with high school level physics but no-one else is replying so this is more of a bump)