VPN Dsiconnect Problem

I recently setup a “simple” NT 4.0 VPN server using the supplied PPP Tunneling protocol. I have (2) remote sites that login using DSL. These sites have a drive mapped to the server and they run a DOS app (sorry!) via the connection. Everything has been working fine, but my customer tells me that his VPN connection consistently drops after about 15 minutes. The Internet connection used stays up just fine, but the VPN connection has to be re-initiated. This can happen in the middle of the DOS app. Does anyone know exactly where in NT I can change the VPN connection settings? It seems to happen at about the same interval, leading me to believe it is an NT or client setting. The clients are running Win ’98….

Thanks