We've encountered this issue at two clients running 2012 R2, although it may apply to other versions of Windows Server. Add the following registry entry, or run the attached file to limit the search indexer core count to 1. For whatever reason this fixes the issue.

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Search]
"CoreCount"=dword:00000001

Reference

Attached files:

• Search-Indexer-2012-R2-Fix.reg