No idea. xD The set doesn't look too old... Does it help when you slap it? Might as well be a bad solder joint or a plug that needs reseating. I doubt this unit still has a (scratchy) potentiometer for +B adjustment 🤔
In any case my #1 suspect would be something in power supply / +B regulation.

Sony use "M" for microfarads in their manuals, I've also seen a legend in some manual where they state that "pF = MMF" which is even more cursed

Hmm, could be a +B drop which would result in a couple of secondary voltages dropping, such as deflection, anode voltage, screen, and focus. Vertical deflection seems unaffected but it might be derived using a downstream regulator with enough headroom..
Looking forward to the results of the recap =)

If only it could work with HDMI switches 🥲

It had quite a unique mod - two buttons on the front allowed you to flip the picture over, reversing deflection yoke polarity. Eventually I removed it because it caused some trouble (spontaneous collapse and contrast all over the place...)

Yup! Bought two of them off eBay and each was modded in some way. The one buried under the desk was sadly shipped in a cardboard box with *no* padding at all and got beat up badly, bezel cracked, tube was basically loose inside the case - but survived. (Terrible burn-in though. ^^;)

They like to hide under tables and desks, it seems