This guy has caffeine pouring out of his skin, but in this video he shows one way to discharge a CRT:
https://www.youtube.com/watch?v=YfQbdvB8GSM
Warning: His intro is painful, so turn the volume down before hitting play.
Something I'm not sure if he mentioned - a CRT can recover a residual charge after sitting for a while, so it's a fine idea to re-discharge it if some time has passed and you're about to touch something dangerous.
I suppose you could even use a cheap multimeter to verify that it's discharged, understanding that if you're wrong, the multimeter will be destroyed. 😀
I think the pros prefer to use heavy duty resistors that drain the charge more slowly, probably because it's safer for the monitor. Many monitors will self discharge if left unplugged, but you shouldn't rely on that.
If you find yourself working near any capacitors, you can drain those by bridging the 2 leads with a screwdriver.
There's probably other youtube videos about the same topic, might be a good idea to get multiple perspectives.
Bad solder joints are common in CRTs, and that link from squareguy definitely reinforces the possibility.
If you don't find the solution that way, then I've also frequently heard of capacitors being a culprit with geometry problems. But I have no idea whether that actually fits this symptom or not.