Most likely its a warmup fault in the focus voltage area.
You could try adjusting the focus pot in case its just a dirty pot, but
BE CAREFUL, you can kill yourself inside a monitor with the case off.
Leave it on for 30 or so minutes and then adjust the focus. Its
possible too if its an older monitor that the tube is dying -
specially if the picture quality generally isn't that good. (ie: lack
of brightness etc). turn the pot back and forth a few times too as
this might help "clean" the wiper and carbon area.Most monitors made in recent years the focus (and screen) pot and
resistors are sealed in as part of the EHT assembly and you can't
clean pot contacts, replace parts etc.Cleaning of the spark gap for the focus circuit on the neck board may
possibly help too. These can get dirt and crud in them - and while
its not likely to be the problem - its worth a try. MAny years ago I
did see one of these so filthy that it did occasionaly arc across
quite loudly.
