There could be several issues causing this. If you've already tried the basics such as restoring (which i doubt will help) then you should check to make sure the wifi grounding chip is in place along with its corresponding screw. They're located at the top inside of the iphone where the 5 screws go together to hold down the plate covering the lcd/digitizer/power button flex cables. The top LONG screw needs to be in place as well as a tiny seemingly pointless piece of grounding metal on the back of the plate cover. I cant seem to find a picture but if i do, ill post it for ya here