Could be that the Degaussing Positor is U/S. This works briefly when you switch on to pass a high current through some electro magnetic coils around the CRT to remove natural magnetism which would build up and spoil the colour convergence.
You had best take the TV to a qualified repairer. Not DIY recommended.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix televisions.