title: marks I have set are deleted periodically
body: <p>My marks are randomly deleted while using gvim on my ubuntu machine.</p>
  
  <p>For example, if I make some marks with <code>ma</code> and <code>mb</code>, tab away to firefox on another desktop, come back to gvim later and try to navigate to the marks <code>'a</code> and <code>'b</code> I get the error:</p>
  
  <p><code>E20: Mark not set
  </code></p>
  
  <p>What could be causing this to happen?</p>