An abnormality with CP 342-5

Hello

We have a strange problem with our CP 342-5 modules. It sometimes goes to a not responding mode with its four LEDs lighting (neither Off nor Flashing)! Afterwards, there would be no access to its slave nodes and we won’t get rid of the problem by stopping/running the CP, resetting the CPU and even disconnecting the 24V power of the CP! The only solution is switching the PLC off and then on. What’s the matter with CP? What situations may bring about this irregularity? If you need more information to be able to respond, please let me know.

Thanks for your consideration and time