Sounds to me like you may have a defective head unit if it only cuts out when you change channels. If it cuts out all the time then your antenna is probably not mounted correctly.

The antenna needs to have a flat area of about 6 inches around all sides (if i remember correctly) and an unobscured view of the sky. Major metropolitan areas like DC also have a terrestrial broadcast to supplement the satellite signal so your XM coverage in DC should be very good.

Either way, I would say that you should go back to circuit city and ask them to fix it.