I'm stumped and I've tried almost everything!

So my problem is really confusing and problematic. I started having this problem with my iPod Touch 5 about 2 1/2 to 3 weeks ago, where my iPod stopped connecting to my WiFi from certain areas in the house. (i.e my room, the upper floor and lower floor above and under the floor my router is on) It did not let me connect whatsoever, and I've never had this problem before. It would either tell me the password is incorrect (which it isn't), or just ask me to put the WiFi password in over and over and over and over again. I would have to go down to the room where my router is to actually have it connect. Even if I'm in the living room, next to the room where my router is, it would most of the time connect and show 3 bars, but none of my apps or anything with my WiFi would load. I turned it off and on, reset my network about 10 times, called Apple and have them run a diagnostics, and they couldn't get anywhere, I even WIPED my iPod, and nothing. None of my other devices have had any problems. I took my device to the apple store recently, and they told me to change my router from WEP to WAP2, so I attempted to do that, and my whole WiFi went wonky, everything disconnected, my WiFi network stopped showing up. So we changed it back to WEP, and everything slowly went back to normal, except my iPod. I'm running out of options, and I'm stressed, frustrated, confused and this has all happened on my birthday, and I'm feeling like crap. I've got no idea what to do, and I can't afford to replace it. Any suggestions? Has anyone had any experience with this? Please Help!