First upgrade to latest firmware and before doing so, please review release notes on upgrade paths and if not sure, check with the support department.
As per your limited problem description, it looks to be device specific to me however would like to suggest on following to be tried out after you have upgraded:
If upgrade does not help and no configuration has been done to disconnect after 5 mins then please create a test WLAN with no encryption/security to see if you see the problem or not.