I often set
Settings -> Sound & Display -> Screen timeout to
never timeout. When I turn off the device the settings is
1 minute again.
Do you know how to make the settings durable? My version is 2.2 (Froyo)
Try this if you added apps and somehow changed some obscure setting. Nothing to do with firmware, whoops.
Setting > About device > Repair and formatting tools - Delete device configuration and installed applications
Failing that or if you're using it for reading & something similar. You should try one of those apps like "Tasker" where you can prevent screen dimming events during other apps of your choice. There's dozens (possibly hundreds) of apps that will keep your screen active.
If you haven't already, go here to Customer Service & contact the professionals that are paid to provide you with support. Perhaps this is a defective tablet.
There is a thread on archosfans.com about the precise issue you're having: http://forum.archosfans.com/viewtopic.php?f=47&t=34624&view=previous
I did a complete restore it started saving the screen timeout setting.