Losing connection to AD

I have been running openfire and spark version 3.9.3 since may, installed on my primary DC

it works great, most of the time, but at least once a week my users lose connection to the server, and I cannot login to the admin console

basically, it cannot connect to my AD users any longer, until I shutdown the server for a few minutes, then restart it

any idea why this is going on?

thank you,

John