Ok, I think I found the pb at my end.
I optimised as much as I could my first world (101 draws in the biggest level, 7.7 mo of ram, 230 ko), that I decided to authorize myself a big image for the background (1024), to do nice clouds moving with the scrolling.
As I tried to get rid of everything one by one to find the pb, I tried my game with a 512 image, and the pb disappeared.
So i thought, ok, it's that, even if the Iphone 5 is supposed to handle that very easely, but as I'm naughty (lol, sorry english isn't my first language, don't know how to say that), I tried with the same image but at 1020.
And at that size, the pb was not there neither.
So I tried at 1023, and... it works !
I have now a constant 62 fps on my Ipad2 and Iphone5 (just once 58 on the Ipad2, but with no "burp" like before").
It's the same than before in my Ipod4 (between 40 and 60), whatever is the size of the image (1024 or 512, I think i'm more "stuck" with the number of objects on this device), but it's reaaly playable, even if the FPS do a kind of constant up and down.
So at the end, don't know if everyone here was using a big image, but it seems that the trick is simply, if you want to use a big image, to use a 1023 px image instead of a 1024.
Personnally, I will finally use a 512, cause the graphic difference doesn't justify (for my project) the differecne of size, and cause it will make me win memory (8.3 instead of 10.2 for the moment), what can be good when i will have my entire game (i'm still finishing the 1st world). Maybe I should try 511 too haha.
That's all !