Having Iphone problems?

Ive had problems with my 3gs since last night. I get full bars of 3g and everythings fine in my house. But when i do things like send a text or anything data related,my bars drop to 0 and it says searching then no service. I have great service in my area and in my house it never done this untill now. I went to apple and they even replaced the phone, this only happens at my house and sometimes other areas for some reason. It continues to happen on the replacement phone they gave me. And right now im restoring my new phone because i just restarted it and it wouldnt power on. So what can the problem be with signal dropping? will i need a new sim card, and how will i get a new sim card from at&t. I dont want to go through activation again with itunes and renew my contract. Is there a way att can just activate the new sim there?thanks