Small dormitory wireless routers can not coect to internet

To all experts in network engineering,

I got some weird problems occurring in a small dorm house (30 rooms total) inteet coection that i currently in charge to manage, i couldn't figure out the reason yet. I do know a little bit knowledge on computer networking.

Before posting about the problems, here is the current network layout of the dorm:

http://i.stack.imgur.com/6YFvV.png

Take in note that I already did some experiment last week (for the entire weeks), here is the data/conclusions:

1. Wireless Router A always in good condition, it means that any device that coected to the Router A wirelessly always able to coect to the inteet, while other device that coected to Router B, C, D might have problems when coecting to the inteet.
2. I did some experiment on Wireless Router D, since this is where the most complaint (problems) occur, look at the picture, Router D coect to a PC and other device (wirelessly), sometimes the pc (coected via etheet) has inteet coection while the mobile device (coect wirelessly) not, some other times the mobile device (coect wirelessly) has the inteet coection while the pc is not, happen most both PC (coected via etheet) and mobile device can not access inteet.
3. The problem occur only when all the resident at the room, due to the fact that they have all their mobile device (laptop and phone) and sometimes they use their mobile device to share (tethering) inteet coection to other devices.
4. During most working hour when resident is not in the room the all network works like charm (including all device coected to Router D).
5. Sometimes restarting/rebooting (not reset) Router A would solve the problem for like (literally) 2 minutes, before router D going no inteet coection again.

Question :

1. What is the possible cause of the problems?
2. How to fix this?

   ADDITION

   Router A : FiberHome AN5506-04-F2

   setuprouter.com/router/fiberhome/an5506-04-f2/login.htm
   
   

   Router B : Movistar ASL 26555

   adslinspain.com/modems/movistar-homestation-amper-asl-26555/
   
   

   Router C : TP Link TD-W8101G

   tp-link.com/en/download/TD-W8101G.html
   
   

   Router D : TP Link TD-W8961N

   tp-link.com/en/download/TD-W8961N.html