Problematic Tek 7A26

I have a 7A26 vertical amp unit that is misbehaving. The symptoms are as follows:

(1) The amplitude of a given signal does not remain constant when trace is scrolled from top to bottom on CRT.
(2) Both channels behave as described here.
(3) Occurs with low frequency traces (10 KHz) AND high frequency traces (10 MHz).
(4) A signal that shows 3 div is positioned with the upper peaks at the topmost graticule line.
(5) Its position is lowered using the POSITION pot to the next lowest graticule line and still shows 3 div.
(6) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 2.6 div.
(7) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.
(8) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.
(9) Its position is lowered using the POSITION pot to the next lowest graticule line it shows 3 div.

The rapid motion sequence of the positioning is that the trace size diminishes in the upper portion (at approximately the second graticule line down) briefly and then regains its amplitude.

The pictures of this sequence are shown below: