Weak GPS Signal Signal

I had this problem with my phone, but in my case the issue were the solder joints in the motherboard (PCB). I took it off and put in the oven for 7 minutes at 190 C, then, my GPS started workin' great(the oven's heat can melt and fix cracks in it).

But in your case, as the GPS was fine before disassembling it, my guess is that should be something connected wrong. You changed the screws, but did you put then in the right place? You can't exchange them, every screw has your own place. Check the Wifi/GPS antenna connection (close to the battery connector) too, maybe it's not well fixed. At the top cover of the phone are five screws, the 1.4mm Phillips (step 7 - http://tinyurl.com/43aon77) has a nut under it that use to fall when we make some movements with the mainboard. If that happens, the contact between the screw and the PCB is lost. Try to check it too.