Usually my O2 sat is 91, at night it drops down to 87 and waves back to 91, sometimes I get 96 for a few minutes, all these observations while resting, why is it so unstable? It seems there are periods of time (hours) when it is better but I don't understand why

If I try hard I can get up to 98 for a couple of minutes but never 99!

Doctor is slooowly testing what could be the root cause of my low O2, I'm in my late 30's, sedentary, with little overweight and I did have pneumonia 10 years ago, I was a social smoker but quitted 15 years ago

I also have OSA but my O2 is wavy when awake!

Please help!