Redundant CPU S7-400H one CPU warm restart occured and lose of redundacy

We have 2 redundant CPU S7-400 H, alarm from PCS7 HMI for CPU error in PLC rack 1 , Synchronization between CPU's is not bossible , PLC is runing  in solo mode , Synchronization module IFM1F/2F fault .

We checked the diagnostic buffer for each CPU and found theat CPU performed warm restart , the time between alarm coming and going is 1:13 minutes , diagnostic buffer is attached 

Visual inspection performed for PLC rack and nothing abnormal detected 

What is the possible causes for this failure ? And what is the action required to eliminate this issue to keep redundancy between CPU's 

Thanks