Rob,
Not sure if your issue is the same as I had or not but a least sounds similar.
I have 105 shifters as well
here is the thread I started a few days ago and solved yesterday
http://www.bikeforums.net/bicycle-me...ur-issues.html

The issue was the way the shifter cable was attached to the derailleur... how it was in the clinch bolt

I can post a pic if needed