DS87C530

Would anybody know if there is a physical fault with the Dallas 87C530
which would cause the device to need reprogramming after a couple of
months of runtime.
I have a several devices running the same code, all use battery
backed-up internal memory, watchdog timer enabled, frequently
addressing external devices via latches, and making use of both serial
ports.
The problem is that after a few months out in the field it seems that
they 'lock-up' and a power reset is not enough to solve the problem. 
They also need to be removed from the battery backup or erased and
reprogrammed.
I did have many code problems in the past but I can't think of
anything else other than physical failure.
Can anyone shed light onto this problem?
I use the Nohau programmer but have had to throw away a large amount
of the OTP versions since they wouldn't fill making me suspect the
device.

Any help would be appreciated,
Jason Dexter