Naidu,
Did you check the CPU utilization for the appliance? ACS-->Reports-->Appliance status
Since this is happening even after reimage so we need to check the acs logs. I would suggest to increasing the logging to full (if not done) and wait for issue to reoccur.
System Configuration --> Service Control --> Level of detail - Full At this point, we need to duplicate the issue.
Open a TAC case as we need to check these logs in detail.
Let me know if you have any question.
Regards,
~JG
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