Did you ever receive a diagnosis? I have been dealing with the same thing for over a year now. I did an overnight sleep study and was diagnosed with nocturnal hypoxemia....I was at 88 for about an hour. They put me on supplemental oxygen for sleep and it seems to be helping, but I do still get dips in my o2 levels in my sleep....sometimes as low as mid 70's. I did a pulmonary function test and it was normal, but when I walked a few laps in the office with my dr and had the o2 reader on finger he could see that I would have sudden random drops...as low as 93 and he wasn't expecting to see that. I get that same feeling of my chest having this kind of sqeezing feeling or heaviness and then lightheaded. The lightheadedness is what lets me know that its happening. My gut feeling is that it is neuromuscular or something with my nervous system. Its like something is short circuiting. I saw cardiologist and my heart was fine, but I do get mild arrhythmias and have bradycardia. I would just like to be able to help guide the doctors to the right tests I guess. It is the weirdest thing and does not make sense to any of the doctors. They thought for sure I had obstructive sleep apnea, but I "shocked" them.