We are a small, distributed company with a Windows Server 2008R2 installation. I would like to set up a way for our employees to connect securely to this server via VPN and then be able to map a network drive.

I have gotten this to work somewhat by installing the Network Policy and Access Services Role on the server and using the default settings. I have also created a network share on the server.
The problem is that our connectivity is sporadic (sometimes the service stops listening on the port or simply refuses to authorize correct credentials) and slow.
I can always connect through VPN, but mapping is problematic.

I would be grateful for the answer on how to accomplish this as well as some guidance on whether I am on the right track.
Thanks in advance!