Getting repeated index errors on Exchange 2010

I'm seeing repeated errors in the event log on my 2010 exchange server:

Content Indexing function 'CISearch::EcGetRowsetAndAccessor' received an unusual and unexpected error code from MSSearch.
Mailbox Database: 2010_DB3
Error Code: 0x80043629

This appears for each of the 4 databases I have every 10 minutes or so.  I've done the following which worked for a day but it showed back up again this morning:

Run Repair-ExchangeSearchSymlinks.ps1
Run .\ResetSearchIndex.ps1 -force 2010_DB3

Any ideas on what the root cause of this is and how I can prevent it?

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Dan