Hold on a minute, spraying WD40 on cracked rubbers at idle is a hell of a lot different to giving the bike a handful of throttle.
I was gonna say air anyway, but this sounds exactly like what my bike was doing when my inlet rubber was folded in half on refitting and letting air in.
As soon as i tried to overtake it just bogged down, but was "okayish" when riding normally.
My money's definitely on the rubbers from your description!
As a possible temporary fix, and also as a fault finder, i would try filling the cracks with silicon or something similar and see if it stops it from bogging down.