do you know what is multilink frame relay ? !
Well it is technique or simply method of combining two or more physical connections logically thus creating a single connection.
So I don't think your multilink framing OFF is related to this problem.
Ok now you says that sometimes it stops forwarding. And also you are in doubt that is it due to your ISP.
Well if you try to open sites on XP box itself in that condition (ie. when there are problems) and it is unable to open there then it is ISP problem.
I have faced some sort of that problem but in my case it was LINUX box connected to ISP and the problem was that their DNS server went down, so to solve that I put a series of DNS server available near my area so no more that problem.
And about linux as NAT box you can either install Firefox and use it for forwarding or you can see
Internet Sharing Linux !
