Dont run that monitor with the bright line like that for too long, that is horizontal deflection failure, and itll cook a line down the guts of that tube.
Check the connections for the deflector yoke to the monitor board, if you dont find an issue there, maybe try swapping the board ( if that is a double screen machine ) and if the issue remains, there is a problem with the deflector yoke. If the problem is gone with the other board, it will be the board. It may be some dry solder joints, or it may need an overhaul.