I regularly connect from my Windows XP box to my workplace over VPN.

If my Windows XP box is connected to the internet over FiOS (via ethernet or wireless), that VPN connection remains solid for days at a time.

If my Windows XP box is connected to the internet over any number of hotel WiFi networks, that VPN connection remains solid for hours at a time.

In contrast...

If my Windows XP box is connected to the internet via Internet Sharing from my Nokia Lumia 822 on Verizon 4G LTE,

then the VPN connection to my office gets interrupted every 5 min 10 sec to 5 min 20 sec.  Very consistent.

And then I have to wait 15-30s before I am able to reconnect my VPN.  Pretty consistent.

During that 15-30s that my VPN won't connect, my Internet connection still seems to be up fine... I left a ping running every 5 seconds from another machine on the same shared internet connection and it never gets interrupted.

Does Verizon do something that breaks VPN connections every 5 minutes?