VPN/Ping in Virtual Box not working with NAT

[ForAnyDay]

Host Windows 8.1; 64 bit; 6 GB memory
VirtualBox v 4.3.14 r95030
Guest Windows XP Pro SP3; 32 bit; 1 GB memory
Have Guest Additions installed on the guest
VM network settings > Attached to NAT

Background: I am diagnosing why VPN to my work network (work network is in a different location) is not working. It works fine on the host computer but it doesn't work on the guest. I think the answer to my PING question here will help me understand my VPN problem.

Internet browsing works fine on either computer.

I am trying to ping the IP address that I use for the VPN server at my workplace. The ping invariably works from the host computer but only sometimes works from the guest computer. On the guest computer, sometimes it will return 1, 2, 3 of the 4 pings. Rarely it returns all four pings. Often it returns none. The usual response time on either computer is 305ms.

The response is the same whether I use the dot com name or the IP address of the VPN server so it doesn't seem to be a DNS issue. If I use the dot com name, each computer behaves exactly as it does when using the IP address.

I tried logging into VPN on the host computer. Then I can ping the server's local IP address on the workplace (192.168.0.xxx). I can ping it from the host and it invariably works fine. Then I ping from the guest computer and it works intermittently. At first it returned 3 of 4 and after that returned none.

Why does the guest computer have such a poor record with the pinging? What can be done about it?

Thanks in advance for any help.

[mpack]

As far as I know, the ping unreliability is in the nature of NAT. I'd guess it has to do with how long the round trip takes. Too long and the incoming filter will have forgotten that the incoming reply is supposed to be redirected to the guest.