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Take a look at the two 6GU7s on the far right end of the chroma board. Are their heaters lighting?
If not, with those heaters cold, here's what's happening: the plates go 'waay positive and drive the (G1) grids of the CRT positive (since the plates are direct-coupled to the CRT grids). The CRT beam current goes into avalanche, producing exactly the symptoms you describe. And this overcurrent is extremely hard on the 3A3, flyback, and H.output tube.
Don't run the set in that condition.
Of course if the 6GU7 heaters are lighting, then the problem lies elsewhere.
If the heaters are cold, it's caused by a broken ground in one of the two ground stakes on the far right end of the chroma board. So re-flow those grounds. This problem was endemic to the CTC-12, 15, 16, 17, and 25.
Even if the 6GU7s are lighting, re-flow those grounds anyway as a matter of routine.
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