Fluctuating Signal?

i've had problem few days now.

i'll using 3g data few minutes.
i'll notice change in speed, sudden drop in connection. still shows 3g four/five bars on top area.
put in airplane mode back. show 4 bars , at&t without 3g. internet still won't work. then, bars disappear , show "no service".

after waiting 10-15 minutes, comes again.

going on?
it's annoying problem, , hate unreliable service.

there fixes this?

 

ask at&t and/or apple it.

 

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