I've been experiencing what might be a similar problem while on my university's WiFi network. It will connect, be blazingly fast for a short while, then essentially drop the network telling me it's waiting for a stronger signal. If I disconnect and reconnect it manually, it'll work for a short while before dropping again. I changed one setting a couple hours ago and, so far (knock on wood), it's been working flawlessly. Here's what I did: Go into the WiFi settings; tap the name of the problem network; tap "edit"; at the bottom of that screen, check the "Show advanced options" box; finally, change "IP settings" to "Static" and hit save. I just did this randomly while trying out different settings to see if I could fix the problem; hopefully it will work long term.