Arduino Mac address troubles

Dear all,

Currently we are having problems with an Arduino Uno and Ethernet shield. The Arduino is connected to a RFID shield, which reads a RFID card and opens a door. All card ID's are saved on a server, from which the Arduino retrieves the ID's to check if someone is allowed to access.

Because the Arduino is connected to a corporate ethernet switch, we can monitor its behavior. The problem we have is that the Arduino sometimes uses a completely different MAC address as declared in our code, which faults our ethernet switch.

We have used the MAC address provided on the sticker of the shield and this system works almost all the time. But at very random intervals, the Arduino switches to another MAC address.

Could this be a problem of the shield, the Arduino or something different?

Thanks in advance