Derek are you having this issue? I've got a module that monitors DotNetNuke and all integrated software (modules, skin objects, etc) for any types of performance problems.  It can resolve most of these issues, if not help report their root cause.   I've used it with great success in a few companies that use DNN on web farms.  Right now I'm making additional improvements and will release it to BETA very soon.  Let me know if you want a copy of it.

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Professional DNN Extensions, custom solutions and mobile apps since 2003.
www.OnyakTech.com

I'm getting this also. And it is not a SQL or DNN performance problem. It is something else.

AssemblyVersion: 5.5.1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: 861077b5-87ad-4aa4-864a-0ecec5457e64
InnerException: This operation returned because the timeout period expired. (Exception from HRESULT: 0x800705B4)
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Threading.ReaderWriterLock.AcquireWriterLockInternal
StackTrace:
Message: System.ApplicationException: This operation returned because the timeout period expired. (Exception from HRESULT: 0x800705B4) at System.Threading.ReaderWriterLock.AcquireWriterLockInternal(Int32 millisecondsTimeout) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.RemoveFromScheduleInProgress(ScheduleItem objScheduleItem)
Source:
Server Name: SERVER322

I am also getting this error on a brand new install of 5.51.
AssemblyVersion: 5.5.1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: a18933c3-4047-49ac-b06b-d5c383400165
InnerException: This operation returned because the timeout period expired. (Exception from HRESULT: 0x800705B4)
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Threading.ReaderWriterLock.AcquireReaderLockInternal
StackTrace:
Message: System.ApplicationException: This operation returned because the timeout period expired. (Exception from HRESULT: 0x800705B4) at System.Threading.ReaderWriterLock.AcquireReaderLockInternal(Int32 millisecondsTimeout) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.get_ScheduleInProgress()
Source: