The problem came back. I tried uninstalling the Centrino Advanced-N 6235 driver and re-installing Wireless_16.5.3_e164.exe, but that didn't help.
Meanwhile, for the work that I do that requires a stable network connection, I will need to tether to my LTE iPhone 5s until someone finds a solution.
I am very disappointed that Microsoft introduced this problem in Windows 8.1 that was not in 8. Small point-release Windows upgrades didn't used to introduce regressions like a network driver dropping its connection. No wonder almost all of my co-workers have switched to Mac...for their work machine.
It is a problem with your Intel WiFi card, look back to previous posts about it, the issue with Windows 8.1 is complementary to the Intel card.