samsung u900 refuses to send messages, URGENT HELP!

I recently bought a samsung soul (u900) off of ebay, and i was fairly happy with it... for the first day! But then it started acting up and refuses to send messages for me! It will send a couple of texts and then will just stop sending them, until i restart the phone, or in some cases am forced to do a factory reset. This is very annoying, as you can imagine. I have noticed that when it is trying to send a message, but failing, there is an icon in the top right corned of the screen saying (3G+), where as when it sends messages fine, that icon just says (3G). I need some help, badly!
Thanks.

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plz i have a prob that after sending two texts any other text is not sent it shows the error try again plz tell me wat to do