One more symptom to add to the list. we are seeing the EventLog table fill up (by fill up I mean it gets to big to fast and the scheduled task to do the clean up times out and fails) and bring the site to a halt.  The schedule exception we get says...

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider:
ExceptionGUID:
InnerException: Add failed. Duplicate key value supplied.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: System.ArgumentException: Add failed. Duplicate key value supplied. at Microsoft.VisualBasic.Collection.Add(Object Item, String Key, Object Before, Object After) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.AddToScheduleQueue(ScheduleHistoryItem objScheduleHistoryItem) at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.LoadQueueFromTimer() at DotNetNuke.Services.Scheduling.DNNScheduling.Scheduler.CoreScheduler.Start()
Source: 
 

 I am not sure if this is a cause or a result of the broadcast polling issues we are still experiencing with the lastpoll date field getting stuck and not updating.

Thanks for any help you can give, thanks.

It has been working for 3 or 4 days now. I think it may have something to do with the change we made to the DotNetNuke.Services.Scheduling.PurgeScheduleHistory, DOTNETNUKE. We changed the frequency from once a day to once an hour. This seems to keep the schedule history table small and not get bogged down, which may have been contirbuting to the failure of the DotNetNuke.Services.Cache.BroadcastPollingCachingProvider.Poll, DotNetNuke.Caching.BroadcastPollingCachingProvider. At this point we are not sure what caused the failure for sure, but it does seem to be related to the scheduled tasks. For now we are happy as long as it keeps running.

Thanks again for all your help.