Seems odd that the convergence would go off on its own for no reason. Did you by any chance move the TV? Changing its orientation (for example, rotating 90 degrees in a room) can mess up the purity, as I learned once when I moved my first CTC-11. The result isn't the same as bad convergence, but it can give you some weird colors.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html
