Notwithstanding the very valid responses from morpheus1514 and MR_WHY, you should also check out the thread in
medhelp.org under the question 'stopping breathing when falling asleep'. In particular, the responses by sonic12 and myself (PMV50) may give you some possible insight into your experiences and may provide some alternative approaches to addressing the problem if (as with myself and sonic12) your physician/doctor comes up empty handed.