Cingular and ATT Signals Fighting

I have Samsung 426 with ATT originally and now Cingular service.
The phone has 2 networks to choose: ATT and Cingular (now are both named Cingular) the signals are fighting each other and I cannot lock it even I manually choose one.
I would like at home to be Cingular since I have a better signal but ATT signal prevails all the time and I have no bars or maybe one and calls getting cut off. Any idea what should I do with this phone to lock it at Cingular signal and if I buy unlock Cingular phone will it work well with my ATT card.

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