It happens sometimes with me too... but when it happens, i just dont breathe instead forcing it. When my body feels the actual need of breathing, it does breathe, usually in the form of deep sighs, about one each minute or so...
For me it seems it happens because the body is SO relaxed that it needs far less air, and so the respiratory rhythm slows down noticeably... but i dont think one would die only because the of slowing of the of breathing caused by weed... unless you be using other kinds of meds, which could make it dangerous...
But if not, i think there is no great trouble in waiting until the body feels the need to breath instead forcing it... even cause when im in this slow breathing state i feel FAR higher and can reach some levels of body and mind relaxment that are simply amazing... i feel like floating... damn... its f*cking damn good... :stoned::abduct: