Msx canon V20

It could be a capacitor on the power supply that is dying. If I am not mistaken that MSX model uses linear power supply (heavy transformer) and old style voltage regulator.

It transforms voltage from mains directly to low voltage(by transformer) AC which is then rectified with diodes and stabilized with large capacitors at the power supply output. If such capacitors fail, the signal originating from the mains (50hz hum if you're in Europe) will leak into the computer causing noise in the video.

Because it's near to the frequency used in the video signal itself it causes "wavy" interference.