Sony KV27TS20 Picture becomes a thin horizontal line - Vertical squashed.

I have a 1989 Sony 27" color TV. After about 10 minutes of viewing the
picture becomes one horizontal (thin) line, everything else is ok. If you
hit it several times in one specific area, it goes back to normal veiwing,
and then intermittantly goes back to the vertical line.

I did some research on the net and one possible cause is a "bad Solder In
Vertical Output Circuit" I don't have the schematic and don't know the
location of the circuity, any help in finding and correcting the problem is
appreciated.

Thanks !!