It sounds like you're getting interference from somewhere. Either disable the MRZ's internal IR receiver, or cover it with something. If you have any IR receivers in the system, temporarily disconnect them (one at a time) and see if the problem goes away. If that doesn't work, start disconnecting keypads.

Also, check all your wiring. Disconnect power to the MRZ, and then power it back up. There's also a possibility a keypad, IR receiver, or the MRZ is defective...