Sputtering at idle, could be caused by a bad glow plug, improper mixture (too much fuel at idle).
If it sputters and cuts out without revving up I would have to think that an air leak isn't present.
Also you said you can throttle up and clear it out which is pulling me toward rich at idle.

Here is a link to the OS .40 LA manual http://manuals.hobbico.com/osm/40-46-65la-manual.pdf
Refer to page 25 for low speed needle adjustment. also you might want to go through an entire retune
as per the manual. I have found that when I get things a bit screwed up a new plug and a full retune from
factory needle positions is always a good starting point.