I have the exact same problem.  AT&T has replaced both my microcell and my phone, and the issue is still the same.  The persons I spoke w/ at AT&T microcell tech support have no further ideas and indicate they've never heard of an  issue like this.

At this point, I cannot use the microcell.  It works fine w/ my wife's iphone, so I just activated the cell just for her phone.  Cannot use it on the captivate due to the battery drain.