you might have an issue with the B+ being a little low....
but more likely you have a capacitor in around the flyback that is getting weak.
the power used to drive the picture from black to white needs to be nice and steady and not vary. if it varies, the you get a weird banding or sometimes in extreme cases you get a screen that starts to collapse in on itself (get smaller) when it gets bright. usually it's a failing capacitor but could be a B+ going all over the place.