Simple, the capacitors in the vertical output circuit are going bad.
Usually the pump up capacitor and return capacitor. Take a look in the
set and find the vertical output ic on its heatsink, usually near the
crt yoke connections, find the capacitors, replace them.

Go he http://www.eserviceinfo.com/equipmen...nasonic_2.html
The ct27D10 should have a similar vertical output circuit so you can
see in schematic form the general, typical vertical output ic design.