Recently we are facing this issue on one of our SharePoint 2010 server. Every morning we restart the service.
I checked below articals, but Service Account is already added in "Log an as Service Account"
SharePoint 2010 Timer" service needs to be restarted everyday
SharePoint Timer Service keeps stopping
I checked the event logs, there is Timer Service Stop event at 6.13 AM in the morning due to Timer Recycle Job but there is no any Start event for same.
I have also cleared the timer cache from server, as suggested in below answer of Andy. Still issue persist.