After updating to 4.3, I'm having issues staying connected to my home and work WIFI. I keep getting the message "Network disabled because internet connection is slow". I have already had this phone replaced once for the same issue. I'm fairly confident it's the device and not the network because other phones are running on the same WIFI with no issues.
You may have too many WiFi networks in these areas that are on the same, or near the same channel. There's an app in the Play Store called WiFi Analyzer that might be able to help you sort this out.