Problem with my computer audio

Hello everyone - I am experiencing a strange issue, and hopefully someone here can help.  I have an Audioquest Dragonfly USB DAC which I use between my MacBook Pro and my amp.  My music files are still currently all in iTunes.  

Lately, when I hook everything up properly, I am getting either no sound at all, or it plays fine for about 30 seconds and then stops.  If I then try to play a different file, it tries to play for less than a second and then the sound stops (the file is continuing to play, just no sound).  I have diagnosed that it is not my amp or speakers.  I do not think that it is a problem with the Dragonfly.  Everything is getting power just fine.  

I should also mention that a while back I was messing around with the demo versions of Audrvana and Pure Music, and I think I changed some settings on my computer for optimal use of those programs (don't think most of them were sound related, though).  I didn't end up purchasing either, though, so I am back to using iTunes for playback.  So I think this is an iTunes issue, though I am not completely certain.  Do I need to change some strange setting back to  normal??  I don't remember what all I had changed.  Right now, the basic iTunes and System Preferences settings seem correct - I have it set to the Dragonfly for output, and the volume is all the way up.  

My computer is also quite full with all the music files, so is this possibly part of the issue??  Has anyone else experienced this problem?  Help?  
Sorry - I should also have added that the files DO play properly on the internal computer speakers, but I would much rather listen to them through my Cornwalls, of course.  

Not familiar with your setup, but I'd try playing with any buffer settings in iTunes, DAC Driver and the MAC OS audio settings.
Lear - Have you also checked the Midi Window (little keyboard icon in tool bar) to confirm output there is to the Dragonfly?
Did you delete Audrvana and Pure Music?  There might still be files from these programs in your User/Preference folders.

Google "how to delete programs from mac" to see how to remove all vestiges of a program.
Thanks for the suggestions, everyone.  I just de-installed Audirvana and Pure Music, so I will see now if that did the trick.  
Nope, still no sound.  Hew, what buffer settings are you speaking of?  I am on a Macbook Pro using iTunes.  Ghost house, I do not see a keyboard icon at all??  

I did go into the settings of both iTunes and of my audio utilities to make sure that those are correct, as per the set up instructions from Audioquest.  It has worked fine in the past, as well.  

Lear - I could try and email you a screen shot to show the tool bar icon I'm talking about.  Send me a message via Audiogon if you would like to proceed that way.