OK, figured it out, the A5 cache is not a problem anymore.
I sorted it out by doing the untether in a single thread and by flushing all the dcache then all the icache in a row at a strategical point of the process.
It took me like a hundred of tests to find the key. Hard for the nerves.
For the tech guys, here is a link explaining issues related to self modifying code ( or code patching ) on the ARM platform : http://blogs.arm.com/software-enablement/141-caches-and-self-modifying-code/
Another news : I discussed with @saurik today about the launchd boot process, and he's found one missing piece of the puzzle I needed to have a perfectly stable jailbreak. He's definitly one of the best iOS gurus out there. Thank you saurik!