CRT, short scale-up distortion after warming up.

[Videodrome]

Eizo T961. (21" CRT)

After warming up for about 20 minutes, this scale-up glitch starts to happen in ever increasing intervals (20mins --> 15mins. etc.)
https://imgur.com/a/aq6eN7k

On the inside nothing seems to spark when this distortion happens.
There is slight a electrical noise, but it feels like a normal 'set resolution' sound.

I just cleaned the anode area+cap and put new dielectric paste + blew pressured air into all pcb's.
Sadly that didn't help.

Probably going to check the solder joints next.
Anyone knows what pcb board to start on? Since it happens when things get warmer, any advice on what components to check?

Any help is appreciated.

[bookaboo]

Dry joints are possible.
The other suspect are any electrolytic capacitors, worth replacing all these anyway.