Connection timed our error

I am trying to have my clients connect to Win2003 FTP
server, thru our VPN tunnel. When they attempt to
connect, they receive a Connection Timed Our error.

If I hit the Internet, my clients can come in and out of
our server at will. If the incoming status is idle for
an hour or so, they cannot connect again. To fix this, I
hit the Internet again. This opens some pipe, and all
can connect. Permissions etc, must be OK, or they could
not connect at all.

Can you help me to understand why my incoming connections
thru a VPN tunnel time out. My ISP says it is not on his
end. We recently upgraded to Server 2003 from
Windows2000 Professional. All worked just fine in 2k
Prof. Since installing Server2003, we have had this
problem.

Also, my ISP cannot ping my 2003 server, but if I hit the
Internet, he can then ping my server normally. All stays
OK until it times out again.

I will truly appreciate any and all help.

Thanks in advance. Charles Topping