Sounds like the degaussing positor may have failed. If so, it's a very simple and inexpensive repair. Usually, you will get a blotchy rainbow like effect to the colours, especially around the edge of the picture, if that is the fault.
There are other possibilities but it does sound like the positor's failed. It is usually safety specified, so the correct type must be fitted, but still a simple job.
Cobweb
July 2006