Aha! I had this "wifi drop out" last night with a specific setup so I decided to investigate this evening.
In my attic, I have a DVB dongle connected straight into a coaxial collinear antenna, and thence to a 5m USB lead. It works great with my PC.
I received my Pi B+ through the post yesterday to replace my existing B, to make a proper unit which can take a USB GPS as well (on order). I was trying out the B+ with one DVB dongle and it was working fine here in the house. However, by the time I got it working it was late at night and there weren't many airliners around. So, thought I, I would go upstairs and plug my antenna/dongle/5m USB lead combo into the Pi to see if I could get any aircraft from further afield.
So what happened? Well, the Wifi wouldn't stay on, it kept dropping out and wouldn't connect back unless I rebooted the Pi. It stayed on from a matter of seconds to a matter of minutes, but drop out it did. I tried it with two different tablets. So then I went to bed...
This evening I had another go, and sure enough, same results. When I swapped the DVB dongle back to the one I was using before, plugged directly into the PI, the Wifi stayed up...and it's running up in my bedroom now and has been up for hours.
So it could be the DVB dongle, or the combination of stuff plugged into the USB ports may be overloading the power available on the sockets.
I meant to try using my Pi B instead of the B+ but I haven't got round to it yet.