Signal strength keeps comming and going 32GB 3GS IOS 5.1.1

I purchased an used iPhone 3GS for Rogers wireless network (canada) but my signal keeps comming and going (my iPhone 4 had perfect signal) I want to change the docking port to see if that fixes it I don't believe it's the sim card it's brand new, but it could be, but the last time I changed a docking port on a 3GS the attena wouldn't fit back on properly then the outer ring broke, I googled it and I read that the sticker on the back of the docking system should be removed from the old one and be restuck on the new docking system, is this true I don't want to ruin this phone because I didn't get it very cheaply so pla help with some ideas.