i had the exact same problem with my Apexi bov
you could actually see the piston in the valve opening as the revs dropped then it would close and the revs would rise
it would do this for ages if left idling up until the point where it stalled

I assumed that the valve opening would richen the mixture then the revs would rise

,then the valve would shut and the revs would drop causing a pressure difference across the valve causing it to open again

,and hence it kept happening only slightly worse on every pulse

.Eventually letting the revs fall to the point where it would stall

I tightened up the adjuster on the top of the bov and since then not a problem
my valve

(and others i have looked at) had a 10mm locknut to undo then when it is loose you can then adjust the screw it is on in or out adjusting the pressure on the spring.

Then tighten the locknut up again