CM 800 which is the ancestor of the 3000 loco has the older reversing unit which uses an interim stage or neutral. In other words... running forward, click to stop, then neutral, then click to start in reverse etc. It also has a reversing lever.

I'm familiar with this system, I have it on several other old locos but this one has got me stumped.

I cannot get it to stop 'jumping' whatever I do. It jumps when going into forward or reverse but not when going into 'neutral'. Obviously I have been trying to adjust it with the unit's spring but is there anything else that might be causing me problems...apart from age?

That's completely normal behaviour with those type of reverse units - I've got two CM800s like yours.
They don't switch off the motor when being activated like the later ones do. So when switching into forwards or reverse the motor gets the full reversing pulse voltage hence the big leap.
You can practice flicking the controller knob to give a very brief pulse but it's not ideal.
I think the old controllers used a push down on the knob to give the pulse so it was probably easier to give a very short pulse with those.

The issue is that the reverse unit does not know whether to interpret the surge pulse as a command to trip the relay or to shoot off at high speed.
Most likely, this is because the voltage is slightly below the strength needed to trip the relay. It is possible that due to voltage changes
Try it with a different transformer to see if it works. It may be that you are using transformers that were designed for 240 that now operate at 230.
The result would be that it now delivers only 22.9VAC instead of 24 needed to swiftly activate the relay.



Regards

Mike C