wifi sync cannot find devices in iTunes.

I have been using Ipad2 Iphone4s Iphone3gs AND macbook pro for months, but something weird struck.

My itunes is the latest version till now, and I can find the devices mentioned with USB cable. The point is when I choose Wifi sync, the devices could be seen in Itunes at first, but some time later, the devices could NOT be found, then totally disappear. If I shut the iOS WIFI switch and reopen it, the device may be seen again. Weird? huh?

Does anybody know the reason for that? Is that my Router fault? or IOS bug?

FYI ,https://discussions.apple.com/thread/3415814?start=0&tstart=0 ,this post pointing to IOS WIFI BUG, IS THIS RELEVANT?