Rolling back the firmware is difficult as no official firmware images were ever provided -- just incremental patches.
...
BUT. Ixtab has done most of the work for you. Kubrick (see sticky thread) has a 3.4 image.
As for Duokan -- I would bet it doesn't work because they stole the old update packager which is broken for fw3.4.2, instead of using KindleTool. But they don't want to use our jailbreak cert from KindleTool.

Good luck getting them to fix it.
See the BackDoorLock hack for preventing official updates. Requires our community jailbreak, in addition to the special Duokan-only one.