None of the above suggestions work for me. Likely the real cause is physical and related how Application Loader is using the internet. I have ADSL2 connection with a Netgear modem/router assigning each device at home a fixed IP address according to the mac address of the network interface of each device. I had noticed that when Application Loader is uploading, my Windows PC could not connect to the internet. However, if in Mac mini I use Safari doing various things like watching video, my Windows PC can access internet.
Even stranger, when Application Loader is uploading, the modem/router could easily lost connection to the internet, as I could see from the LED lights of the modem. The connection is lost randomly at any progress of uploading the 20 MB file.
So I had turned off Windows PCs, iPads, iPhones and Android device at home, so Mac mini is the only device using the Internet, the Application Loading has finally been able to upload the app successfully in one go.
My guess is, my home phone line has some noise, and Application Loader is using the internet aggressively particularly the up link, so stressing out the modem, then causing other devices like my Windows PC lost connection, not even able to resolve host. When Safari is uploading video to App Store, no problem.
So my working solution is to make Mac be the only device using the Internet, given that the internet connection is already in poor condition with noise, near the edge of losing connection.