TV picture blanks out, the restores

This is a CRT , not flatscreen TV. After it is on for a while,
suddenly the picture tube seems to go off. It goes black
and there is a tiny bit of light in the center for a split second,
like the power is being turned off. At the same time, the audio
remains, but diminishes slightly in volume. Then, after a few
minutes, the sound come back and the picture returns as well.
This problem repeats and repeats. Now that I am watching
it very carefully, I notice that in the fact the slight drop in
audio volume just barely precedes the blanking of the picture,
just by a fraction of a second. Any ideas? Thaks.