I agree. I don't see how a HV or focus voltage problem would mess up
the focus in only one direction (unless it's some strange interaction
with one of Sony's non-standard shadow masks or something like that,
but that seems unlikely.)

It seems more likely that, if you are somehow losing your video
bandwidth you'd get blurring like this. Something in the video
circuitry could be intermittently limiting bandwidth, so when the fault
is occuring and the electron beam in the tube is about to draw one of
the lines making up the "o" in my example, instead of the video level
jumping "instantaneously" from white to black (which requires good
bandwidth) at the edge of the "o", it rather ramps down to black. So
what you see is a fade (aka blur) from white to black instead of a
nice, sharp, crisp line.

If I've convinced you of this hypothesis, I also have some land for
sale....