We were seeing similar issues with early releases of version 11.2, until we upgraded.
I'd recommend upgrading to at least SR8, but preferably SR 13, which is very stable.
There are so many possible issues, they are too numerous to list. Try checking you QlikView Logs and Windows event logs for more information. Is the QVS or any other Qlik service failing and restarting?
You should contact your vendor support for more detailed recommendations.