Hitachi 50UX22B projection TV picture shrinks top and bottom.

While it is normal sometimes, often the picture shrinks, with the top and bottom full black, while the middle 2/3 to 1/2 (varies) is nearly normal. Side-to-side is normal. Sometimes the picture returns to normal for a while (seconds to minutes). No pops or snaps.

Sometimes the picture is normal on power-up, sometimes not. Turning off and back on may or may not change it.

Picture resembles " ( ) " turned 90 degrees. It is NOT an hourglass shape.

So far I have:
Sniff test. Nothing found.
Vacuumed out the back.
Inspected for burned swollen, or melted components; none found.
Flexed circuit boards.
Reflowed the solder on about half the connections - some were too tight to try.

I have a repair manual and VOM, but no scope and haven't worked on TVs since the the early 70s.

Ideas?