Well there could be a few reasons. First I would make sure that there are no apps that were recently installed, or updated, that could be causing it to disconnect. Either through just messing with the connection (yes some are just made to annoy) or by trying to do things through the WiFi and it keeps kicking it out. So I would check the device.
If that isn't it, then sometimes you need to adjust the WiFi sleep policy on the tablet. It can often cause this.
To check to see if this is the issue...
1. On the home screen of your tablet go to "Menu" then "Applications".
2. Next go to "Settings" and finally "WiFi Settings".
3. On this screen tap the "Menu" button and then choose "Advanced".
4. Now you want to choose "WiFi Sleep Policy".
5. It should show you options, including one for when the screen turns off. If it is set to do this, then the WiFi will automatically be disabled, and go into "Standby Mode", as soon as the screen goes dark. Also there will be options like "Never When Plugged In", so that the WiFi will go down if you plug the tablet in. Go through each of the choices your device gives you in here, and make sure it is not set to turn it off for such reasons. You also have the option to set it to "Never" disconnect from the WiFi.
One of these should fix the problem.
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