Why does my iphone 3gs for tmobile keep losing signal?

I have a 3gs unlocked and jailbroken iphone and it keeps loosing signal randomly, i can't send or receive calls or messages and it will randomly go off. Is it possible i need to jailbreak it again? And at times i turn on airplane mode then off and i'll recover signal for a couple of moments and i've tried turning it on and off as well as taking out the sim card, problem has been on going for 3 days
-thanks

Maybe because tmobile's map isn't that great

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