Problem using GBS-8200 to convert spectrum analyser RGBS output to VGA

[brian_mk]

I have an old (1986) but still very capable Marconi 2380 Spectrum Analyser that uses a CRT display. It also provides an RGBS output.
To allow me to capture screenshots from it, I thought I would try using a GBS-8200 along with a Gracetop VGA->USB adapter feeding into a laptop running OBS.
The composite sync output from the 2380 is 2V p-p ac coupled, so it needed a pull up resistor to 3.3v to work with the TVIA575 video processor on the GBS-8200.
This gives negative going pulses that should be ok for 3.3V logic levels.

Using the initial default settings on the GBS-8200 the image on OBS appears stable but slightly too large in the vertical direction.
When I reduce the size using the GBS-8200 menu buttons, the missing lower part of the image appears but the bottom few lines are unstable / out of sync.
The top and middle part of the display still look ok and remain stable.

I can see nothing obviously wrong with the composite sync signal when viewed on a 'scope.
I also tried hooking the GBS-8200 sync to an internal 5V TTL (inverted) version of the composite sync signal from the analyser (via a 1k series resistor because of the 3.3V TVIA575). This gives positive going pulses. It made no difference to the problem.

The 2380 spec for the RGBS output indicates:- hsync: 15.625kHz, vsync: 48.2Hz, non-interlaced.

Should this setup be capable of working or am I missing something?
Could the problem be connected with interlacing or the frame rate?

[brian_mk]

Here is a screenshot from OBS that shows the problem when the vertical size is reduced.

[brian_mk]

Composite Sync.
Horiz: 50uS/div. Vert: 1V/div.