Opmanager keep going down

Opmanager keep going down

ever since I rebooted the Opmanager machine last Friday, I have problem with Opmanager.
It it very unstable.
I need to restart the Opmanager service. I restart the Opmanager using the StartOpmanagerServer.bat script a few times this morning.
 
which log file should i check to see what is the problem

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