Disconnect -- But not, Contstantly

I've got a really weird problem and it's driving me nuts.

I use dual dialup modems (yes, I don't have broadband, and it sucks) with
Windows 2003 Server. In both RRAS and RAS with ICS I get the same thing.

When there is heavy load on the connection, (i.e. multiple HTTP links,
downloads that are log etc.) the connection stops working. That is to say,
both modems show as connected in RRAS or RAS and data still is getting sent
out over the connection, but nothing comes in, and if you put a phone on the
line, there is no modem or connection, just dial tone.

My ISP did a watch and it looks like Windows 2003 Server is requesting a
disconnect and then not seeing it on my end.

Anyone else seen this or have any ideas? I can't see anything weird in the
logs...

Thanks!
James Hancock