Re: Weird problem...
does it happen when you recoal or just randomly?
sounds like your getting something that is burning more so if it comes and goes.
i would try cleaning though, it can never hurt (i like to clean my rig before every session, its a mya though so it needs a bit more of a scrub that a smooth bore hookah)
also do you get a bit of white/lighter color near some of the holes on the foil? if so ive figured out that, that means you have to much heat at one spot (only happens with full size coals that are pretty fresh)