Zeno's right. Those 2 ground stakes on the far right end of the chroma board need to be re-flowed. They're Schroedinger's stakes and exist in one of two states: either cracked or gonna crack.
When that happens the 6GU7 heaters go out, initiating a cascade. Plate voltage rises, and since the plates are DC-coupled to the CRT grids, CRT beam current rises, dragging down the HV till raster blooms and fades out, stressing the bejeebers out of the 3A3 and flyback.
Re-flowing these should be done as a matter of course on every CTC-15, '16, '17 and '25.