We have 4.9.0, after a rollback from 4.9.1 due some other unexplained issues.  Now, after the site has been up for about 1 week, we start having authentication issues and this gets logged in the event viewer:

AssemblyVersion: 04.09.00
PortalID: 0
PortalName: Provident Intranet - 04.09.00
UserID: 1273
UserName: PROVIDENT\user
ActiveTabID: 163
ActiveTabName: Home
RawURL: /DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx?tabid=163
AbsoluteURL: /DesktopModules/AuthenticationServices/ActiveDirectory/WindowsSignin.aspx
AbsoluteURLReferrer:
UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; prov_id:830; prov_id:064; prov_id:057)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 567685f2-b425-4faa-8171-08beea791596
InnerException: Thread was being aborted.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.DirectoryServices.Interop.UnsafeNativeMethods.IntADsOpenObject
StackTrace:
Message: System.Threading.ThreadAbortException: Thread was being aborted.at System.DirectoryServices.Interop.UnsafeNativeMethods.IntADsOpenObject(String path, String userName, String password, Int32 flags, Guid& iid, Object& ppObject)at System.DirectoryServices.Interop.UnsafeNativeMethods.ADsOpenObject(String path, String userName, String password, Int32 flags, Guid& iid, Object& ppObject)at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail)at System.DirectoryServices.DirectoryEntry.Bind()at System.DirectoryServices.DirectoryEntry.get_AdsObject()at System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne)at System.DirectoryServices.DirectorySearcher.FindAll()at DotNetNuke.Authentication.ActiveDirectory.ADSI.Search.GetEntries()at DotNetNuke.Authentication.ActiveDirectory.ADSI.Utilities.GetGroupEntriesByName(String GroupName)at DotNetNuke.Authentication.ActiveDirectory.ADSI.ADSIProvider.GetGroups()at DotNetNuke.Authentication.ActiveDirectory.UserController.AddUserRoles(Int32 PortalID, UserInfo AuthenticationUser)
Source:
Server Name: xxxxxxxx

Anything I can try to stop this from happening?  The only thing that seems to restore peace is rebooting the server.

That's exactly it.  Everything works great for a while, a week or two, and this starts happening.  Rebooting the server corrects it for a time.  I've looked at the settings on the HOST admin pages for caching.  Since we are only using AD authentication, what would the best setting there be and what exactly does that do?

There aren't a lot of roles in DNN but users are members of a lot of AD groups.  This started again this morning, hence my visit to the forum.  I checked the memory use on the server and it looks fine.  Processor use is normal.  I can't seem to target why suddenly it would start timing out.  Is there a temporary workaround besides rebooting the server that I could try that you know of?