Rough idle gone at high altitude?

[cx9 2008]

I figured out that when I went on a trip to a much higher (altitude) city than where I live the engine (2008 CX-9) was running butter smooth, no rough idle, no missfire nothing and that was so crazy. I didn’t believe it, then I went lower in altitude and it started to idle rough, I went higher again and it ran so smooth at idle it felt like it wasn't running but it was. This is making me go crazy!

Can somebody please make me understand what happened..?

 

[N7turbo]

Octane requirements decrease with altitude. You may have knock. Have you tried running completely empty and putting in a tank of 93?

 

[wlong01]

Check your engine air filter, make sure it's clean/clear.

 

[cx9 2008]

Octane requirements decrease with altitude. You may have knock. Have you tried running completely empty and putting in a tank of 93?

I always run 95 octane, but the confusing part is that why does my car idle perfect at high altitude but idle rough in lower attitude.
I have never heard of such thing in my life but there you go, once a lifetime situation .

 

[cx9 2008]

Check your engine air filter, make sure it's clean/clear.

Its new and OEM Mazda.