Our Sharp television went pretty much the same way about 18 months after it was purchased.
A purple ring would form and fill the screen. It would get bigger and darker, the longer you watched. Eventually, you would have to turn it off and let it calm down before starting all over again.
It took a TV repair man almost 3 weeks to tell us it was broken and not repairable. But a new 37" HD-ready Panasonic TV in its place, I'm not too bothered anymore.
Our old sony turned the picture into red haze even though you could still see the picture. Got a TV bloke in and he said the tube had gone and to replace it would cost near enough the price of a new tele. TV was only two years old and cost us �450 and I havn't bought a sony product since.