Anyone seen ATMega8515 lose eeprom data?

A customer of mine recently changed from using AT90S8515 to ATMega8515 as the former is obsolete. 
We've had a couple of units in the field lose eeprom data - we're still waiting to get them back to
look further but has anyone else seen anything like this ? 

"Mike Harrison" <mike@whitewing.co.uk> wrote in message
news:qtkcn01h7j6r4m1l7b2tvvnq0q4odditbd@4ax.com...
> A customer of mine recently changed from using AT90S8515 to ATMega8515 as
the former is obsolete.
> We've had a couple of units in the field lose eeprom data - we're still
waiting to get them back to
> look further but has anyone else seen anything like this ?

Yes. I had the same problem. The solution in my case was to enable the
brown-out detection. On boards with just a ATMega8515 the problem occured
every now and then, while on a same type of board with a Bluetooth module
installed, it happened very frequently. I could pinpoint the problem to
occur on power up/down cycles, were the risetime ov VCC was mostly quick
enough without the BT module and just too slow with the BT module installed.
Enabling the brown-out detection fixed it.

Meindert