Probably the monitor chassis is quite old and the capacitors are all dried out.
Once it warms up you get the distortion.
You could also have a dry solder joint somewhere that is causing it, as it heats up the conductivity gets worseand so does the picture.
You could try replacing the caps or just go to Jomac and get a new replacement chassis.
Just be very carefull when disconnecting the chassis from the picture tube as there are VERY high voltages present and they will give you a nasty zap.