The problem is your phone and it sound like a problem with the power amp.
The power amp is the chip inside the phone that controls the RF/TX.
I wouldn't suggest you take the repairs in your own hands as this chip is really delicate and if you don't know how to, consequences are your phone will became useless.
Take it to your local phone repair guy and let him carry out the repairs.
www.themobilephonesurgery.com if you are local