Problem with the automatic box, or not?

My 150 still has the original BW automatic transmission and I only have a vague understanding of how it works!
Some years ago I replaced the original distributor with a new one that incorporates an electronic ‘module’ which replaces the points and condenser. It was fine, made the car much easier to start. Then the module failed and the supplier gave me a new one.
After I’d fitted it the car didn’t start so easily. More importantly, as soon as I put it into gear the engine died straight away. I found that if I used the throttle to get it idling at about 1200 rpm, instead of its normal 750 rpm and then put it into gear, engine speed dropped a lot but it didn’t die and was ready to go. All of this done with the foot on the brake.
So is this a gearbox problem or just another fault with the ‘new’ electronic module?
My take is that when you put it into gear with the brake on, the engine should obviously still run, it’s just churning over the oil in the torque converter. So if the engine dies at normal idle speed it means that the engine is so low on power at this speed that it can’t overcome the drag in the torque converter. So it’s an ignition problem not a gearbox problem.
I know the easiest test is to replace the distributor but I’d like to understand what’s going on first!
Any opinions wecome please
(Posted on XK and Saloons forum)
Bramshill