For a downgrade, you have to follow this tutorial
After the tutorial, download the 2.1.08 firmware here: Klick!
Then copy the file to your Archos and install the 2.1.08.
The only stupid is, that all your apps and saved files will be deleted, so you've to backup them.
The functionality is the same. The 2.1.08 firmware is more stable and the battery loads to 100%. The battery life is more than 30% better, I tested this on my Archos 70.
I recommend the permanent root (Klick!
), because you can delete the Archos updater, so you don't see the notifications of an update to 2.3.26.