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Can't say I've experienced those things with my Captivate, although i have dropped calls at work, but all cell service is marginal here. At home it is rock-solid.
If you have unreliable service at your location AT&T might have a failing cell tower in your area forcing the calls to jump more frequently, although they'll only admit it if it goes completely down. Do you know anyone else on AT&T with a non-Samsung smartphone? Even an iPhone will do for testing, even they notoriously report stronger than actual signal strength.