If its a CRT then i would say you have a wandering cathode ray or steering coil.
This can be caused by a number of things, usually a dry solder joint on the cathode contacts on the small square board connected to the back of the tube.
Other cause can be a malfunctioning transistor which you would find 3 of also on the small square board connected to the back of the tube. these control the (R)red (G)green (B)blue output from the cathode/anode.
Or it could just be that the screen has had a bad knock at some point and damaged the phosphor coating. badly damaged phosphor would not normalise with a degauss.
MY ADVICE: TAKE IT TO THE SHOP!
Please be aware that you should not open a CRT monitor and fiddle unless you really know what your doing as they can hold around 25,000volts!!!!
