I experienced the same issue on my own iPad mini.
Very frustrating, booked into Genius Bar who were very helpful.
My device even disconnected from the stores own network whilst with a support technician.
This was enough for them to agree that the device should be replaced.
Sad thing is, got the replacement home and it does exactly the same as the original device!
Could it be my wireless router...maybe...but why is my iPad2 and Mac Book Air not having the same issues?
If I stay in the same room as where the wireless router is then the iPad mini doesn't disconnect.
If I move into the next room which is no more than 5 metres away, the signal strength on the mini is up and down like a yo-yo and then disconnects! At this point I have to go into settings and manually connect again.
A friend said something similar about his iPhone5 at his own home.
Got so fed up that he purchased an AirPort Extreme and says all is good.
Perhaps newer Apple devices only like their own WiFi routers!?!