picture problem
 

I'm having another problem with my 27" Zenith Sentry 2. The picture
tube only shows a narrow slit of light across the middle of the tube.
This is evidently part of thr picture it should be showing.
Occasionally the whole picture will flash on for a few seconds, bright
as ever. This started quite sudenly last week. Could someone tell me
what the problem is and is it worth having fixed? Thanks in advance.