Zenith Chromacolor 2 Question

[jstout66]

I have an odd intermittant problem with my 1976 Chromacolor 2. Once in awhile I will notice a slight distortion in the picture. It's like the horizontal gets "shakey". It doesn't ever lose horizontal hold but will have a slight horizontal wave in the picture. It doesn't do it often. Just often enough to drive ya nuts. What should I be looking for??