Lurching problem and I'm out of ideas.

I believe a mid 1980s A999 is controlled hydraulically by the valve body?
Not sure Ive never heard of a hydraulic lockup converter though interesting. I looked into it briefly here is the link. It seems there is a hydraulic and electronic system You could pull the trans pan and see ethey hydraulic system for example. It would still avoid pulling the whole unit.

Mechanical override: Lockup torque converters