Search works but items then disappear one by one… Help!

We have one person using Outlook for Mac 2011 (v.14.4.5) and if they use the search tool, the results appear to be displayed as normal. 

However once search is completed, the items found then start disappearing from the screen one by one, starting with the most recent first.

This happens at a rate of about one very half second.

Any ideas?

We've re-indexed the entire hard drive for Spotlight - but to no avail.

It effectively makes any search unusable.