If the problem was in the horizontal output stage, there would not be much (or little) to see (as the horizontal output also delivers the high voltage for the display) and the displayed lines would be vertical.
The problem is in the vertical output stage and is - reading your description - most probably in the connected suppression circuit that should interrupt the display during the short moment of vertical fly back.
First get a service manual. If you don't have one try any of the following Sears links :
Sears on-line website http://www.sears.com/Service http://www.searspartsdirect.com/partsdirecta http://www.searspartsdirect.com/partsdirectb
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Repair : look in the circuit that powers the first grid of the display tube. Brightness control is almost always connected to this grid. On top of that DC signal there are negative pulses for suppression of horizontal and vertical fly back signals. The problem is in the circuitry between the grid and the vertical output stage where vertical pulses should be passed on, but which fails to do so on this moment. Often that is due to either a failing capacitor or switching transistor.
If you have - or can borrow - an oscilloscope : use that. Start from the grid and go backwards towards the vertical output stage to find the problem.
Note : if your television is not a tube type, but LCD, there is little you can do yourself : bring it to a television repair shop ! Normally they do not repair the fault, but replace the motherboard , solving the problem - at quite a cost ....
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