When configuring two dial up lines with each connected to a different ISP, the connection becomes very slow and sometimes we don't get the requested pages.

Because you are using two ISPs it is important that each WAN port of the Proxy Server can use the DNS of the other ISP. When you set this up you must rely on the clients (PCs) of your LAN to have the DNS stored in it's settings. It would not hurt to have the clients statically assigned with a private IP instead of using DHCP. The Proxy Server will dial either ISP regardless of what client makes an Internet request. To troubleshoot this further you could disable one of the WAN ports and see if one of the two ISPs is giving you trouble. Then act on that. The Proxy Server should not be handing out the DNS. It is NOT capable of handing out different DNS addresses to specific LAN clients. You should also have MLPPP disabled.