AVR-Chat

>    Were you attempting to use this non rail-to-rail clock for 
external 
> devices?

Absolutely not.



>    As I undersand that data sheets on this issue, the low power 
(non 
> rail-to-rail) clock option is only recommended when you are NOT 
sharing 
> the clock wqith any other device.


I've confirmed with Atmel that it's a Vittoz type oscillator.
Pull a google on it.  Nice way to save power, but your crystal 
selection is much more critical. Notice that Atmel gives you no 
guidance on what the amplitude should be, or really even that there's 
much 'special' about this mode at all?

This issue forced us into a total recall of a bunch of systems. 
The problems were glitchy, hard to pin down, and blamed on software 
and hardware, up until a new date code of AVR chips arrived, and at 
that point, some fairly large number of them wouldn't start, using 
the same crystal and caps as we had before.