Try this experiment - BE CAREFUL!
with engine at an idle , have a friend help you carefully lift the model so that you can roll it -about 15 degrees to either wing.
see if this cleansup/prevents this funny bobble.
I ran into a similar thing on another brand reed valve engine.
the thing would rythmically surge at an idle .
by simply tilting the engine sideways a bit (fuel tank position is not related), the problem ceased.
This was an easily repeatable test so I got a chance to zero in on it.
Apparantly there was some puddling going on in th engine -very minor but it was there .
The cure in this case was to simply mount the engine tilted to about the 8:00 position.
It may not be true for the DA- but the test is easily done .