Thanks for getting back to me on this...
The output from Motion is a 250 frame 'Generator'.
FCPX does not recognise the video properties of the generator ? so I set the timeline manually to 25fps Apple Prores 442, 1920x1080.
I dropped the generator into the timeline and it automatically renders in just a few seconds...but still plays with noticeable jitter.
I have spent the last 2 hours checking everything I can think of. Trying, by a process of elimination, to pin down what could be causing this....?
The original problem was evident when using a single 3D letter and an attached particle generator, three masks and a base layer of 1080 video...... so your original thought about computer processing power made me simplify what I did .......but the jitter was still there ...... and is still there even when I reduce the computer 'work load' to virtually nothing....? A single 3D letter used without a 'Material' applied, behaves exactly the same as when a 'Material' is applied and there are no masks.
I tried creating an open spline geometry and 'Align' the 3D letter to and 'Move' to get movement along the spline...
'Move' gives a 'strength' control but at slower 'strengths' ( speeds) the jitter is less obvious but still there...?
I am still inclined to think that there is a mismatch somewhere between the 3D letter properties and the frame rate...
I am currently looking for a way of precisely controlling the speed/ movement of the 3D letter...?
I regret I do not know how to get the 'Generator to you ... but thanks for the offer.