Dialup Frequent Disconnects

Computer: Homebuilt; Vista Home Premium; IE-7
Modem: USR 56K PCI Internal FaxModem
Home Network: 2 computers (Vista and ME). Rhine II Fast Internet Adapter.
No internet sharing.

After connecting to Internet, frequent disconnects are experienced. My ISP
says the problem must be on my end, and I tend to agree since it does not
occur on my ME computer. It occurs regardless whether I use my ISP
Connection Manager (apparently a local version of Microsoft Connection
Manager) or a connection created under IE-7. Also, it did not happen with
the two beta versions of Vista; only after upgrading to VHP. After
disconnecting, re-connect is always successful and IE and Mail are normal
and connection
continues until I manually disconect. Also, when I use Vista to create a
new connection, it works fine the first time, but subsequent attempts
display the same problem.

Problem Details: Usually the connection is successful, but lost after
about 3 minutes. In this case the Network Sharing Center shows "Connected"
as does the TaskBar icon. However,
at times the connection is not successful even though everything shows
connected but neither IE or Mail will open. This makes it hard to
identify the cause. Also the IE-7 "Diagnose" function is totally useless as
it says to power down the modem, which means a cold reboot with an internal
modem.
Here are connection log items:

CMMDIAL32 5:36 Connect Event
CMMON32 5:39 External Disconnect
CMMDIAL32 5:39 Disconnect Event Calling Process C:\Windows
Internet Explorer.exe

Two questions: CMMON32 apparently initiates the disconnect. What is this
function? Also, could this be a timing problem, since reconnecting
invariably works? Would appreciate input from any MVP's (especially those
specializing in network problems) or anyone who solved simular problem.