vnc over Mac vpn unstable

I am using an El Capitan server VPN service to connect to my corporate network. Once the vpn is started, I start an VNC connection through the vpn using the standard builtin mac client. It seems to work well at the beginning, but it hangs often and I have to restart the connexion. The connexion will work between a few seconds to a few minutes without any problem and then simply hangs.

If I tunnel through the same server using ssh -l, the VNC connexion will last as long as I want.

Other than that, VPN seems to work pretty well. Nothing gets logged on the server.

Has any one any idea what could cause that missbehaviour ?