Siemens comunication problem

Our system SIEMENS S400/ Redundancy and there are many dreess(power analyzer) has been connected over profibus cable by using db coupler our problem is we loss communication with this drees randomly (stop communication for no apparent reasons), I wonder if somebody has like this problem and any help would be nice

When it happens, what does the hardware diagnostics of the S7 CPU say ?
If the error is active, what happens if power is cycled to the DP coupler ?