My 91 740 wagon speedo (it's a VDO)is acting up.

Odometer works
All grounds and rear diff connections good.
Bulb sensor always lights on startup then will go out after a while.
Rear diff. sensor (combo speed/ABS)works, resistance change with tire rotation as per green book and no ABS lights and ABS works.
When I first turn the key to just before cranking the starter, the speedo jumps to 80 kph (50mph)turn farther and it cranks and starts. Then the speedo drops to half (40kph).
Driving it seems to register correct speed above 40kph.
When the 40 default zero trick is going on the bulb sensor light is on.

I pulled speedo, reflowed all the solder joints, cleaned, tightened and de-oxited all connections at wiring plugs and under screws. No change at 1st.

I noticed after driving for 30 min that the speedo now zeroed when stopped and the bulb sensor light was out.

The link to the bulb sensor isn't perfect meaning sensor light out and speedo starts working or sensor light on and speedo has 40kph zero again. There seems to be a link.

How do I disable the bulb sensor without impacting anything else?

Instead of doing the speedo capacitor fix (if have to) I'm going to a wrecker for another instrument cluster

Any help appreciated.
ian