So if I understand right, the first few shots stink out loud and then it clears up? Is that right? And if it sits for a few minutes it stinks again? If that's what's happening and time is all it takes to change how it performs then it sounds temperature related to me, in which case maybe some rubber needs replaced?
I'm kinda new to airsoft myself, and I'm not familiar with your specific gun, but I used to have this gbb Glock with almost the opposite problem where the first few shots were great but when I mag-dumped c02 mags it'd get cold and no longer seal right.
Art fight. Show us what you got