Lpc2000

Using the Philips LPC 2129, I set-up an I2C slave mode to talk to a 
master device.  The microcontroller, recieves and sends data 
correctly most of the time, but about 1/100 times, when the MC 
recieves:

[SLA][W] it sends a NACK (no acknowledge) back to the master instead 
of an ACK.  In the software a NACK is never sent, so it is unclear 
why the hardware occasionally sends back a NACK. 

Has anyone else seen this behavior?  Could it be an errata with the 
Microcontroller?  I'm not sure what I can do to debug this?  Any 
suggestions?