Sounds like maybe the vertical sweep is not linear. Combine that with the "got better after it warmed up" and it sounds like either the DC for the vertical chip is saging from a bad electrolytic, or the vertical IC feedback electrolytic is bad. Possibly just bad solder joints around the vertical IC, but sounds more like a bad capacitor. The DC power for the vertical section could come from the LOPT, so the cap would likely be near that, not the vertical IC.
This produces a vertical sweep ramp signal that is curved, instead of flat, stretching out the picture at the top.