Does anyone have a schematic or a repair procedure for the guts of the speed interface unit in the trunk of 88-on XJS'? I've done searches here, but not found much.

 

I've not seen a schematic, but it might be worth looking closely under a magnifying glass for microcracks in a solder joint. That happens with older circuit boards over time. Touching each joint with a hot soldering iron can often fix it.