Try this:
1. Open up the Galaxy S3 phone dialer
2. Then dial *#0011#
3. Look for the “ServiceMode” screen and press the left menu button
4. Select “WiFi”
5. You should see that the “WiFi Power Save Mode” button is “ON” – now put it on “OFF”
6. Your done! Just exit the menu, switch off your WiFi, then switch it back on again. If your router details are saved and automatically connects, get your Galaxy S3 to forget or delete the network information. Then reconnect, putting in all the required information.
The WiFi is set to Power Save Mode by default. Most people who do this find their drops/reconnects cease once you turn the power save off.
Mike