Weird problem with Sony KV-20FV12

[old_coot88]

I have a 15 year old Sony KV-20FV12 That's been working perfectly up to now. CRT nice and bright, just like brand new. Then it started a "cycling" act, appearing exactly as if the CRT heaters were being switched on and off regularly. The pic doesn't go all the way out, just fades to like very weak emission on all three guns. Then it comes back up to normal, gradually cycling in and out over about a half minute period. The change is not abrupt, but like a gradual 'breathing' effect.
But the CRT heater voltage does not change, holding a constant 5.2V AC

Anybody had a similar Sony problem?