hmm...we where checking a throttle dody on a polo a while back and noticed that when blipped, the throttle would open and close suddenly,as you would expect, but then would open again, say 10%, then close, then open again, say 5%, or what ever then settle. It seemed to be managing the drop in revs so as not to be too sudden, i didnt make too much of it as with the inlet removed and hence upsetting maf readings it could be doing all sorts of strange things it wouldnt normally. I'm also under the impression smaller engines seem to run on a bit when the throttle is shut so "may" be more prominent on the polo...
But re lpg, your above description made me think of this same throttle action from a throttle body when driving, in that it seems to cut the power instantly and then come back in as if the ecu opens the throttle to manage the revs drop off, in a similar fassion as on the polo.
As i write i realise cable operated throttles would not have this "managment" and presumably still have the issue with lpg as we are experiencing? but am i getting the general idea of the problem at least...sort of ...ish? :-/
Edit to add, if carefull when closing the throttle it seems to improve things slightly...i think...not really played with it enough to be sure, but obviously depends how carefull you are...