Router problems is my guess.
You might try making sure your phone has wifi set to be on even while the phone is sleeping (its in advance wifi settings) then let the phone go to sleep. Then ping the phone from another computer on the same network. If you can't ping it, it means that the phone is ignoring your request to keep wifi on while asleep. You are probably out of luck in this case, file a bug report is all you can do.
If the ping times are really high, but eventually are answered, it means the phone has put the wifi chip set in a low power sleep state. Best you can do with that case is send a google Talk text im to wake up the phone on the other end.
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