The problem you described is caused by poor voltage regulation in the High Voltage power supply. This can be caused by a defective fylback transformer, a defective focus block, a defective HV trippler, of the horizontal output transistor/damper diode assembly. I would first check the high voltage from the flyback to see if it was fluctuating, you can do this with a high voltage probe. The I would check the horzontal output transistor out of circuit with a transistor checker to see if it passes.
If you do not have equipment what you can do is remove the deflection/HV board and order a replacement from panasonic.
Look up Panasonic's web site and contact their technical support number and call them to find out the part number and have them order the part for you. If this fails try PTS at http://www.ptscorp.com
I hope this helps