I am not joining with an answer.

I have a similar problem, I've tried 3 a USR - psi hardware modem, a USR - USB modem, a Conexant pci softmodem.

Either they won't connect or they disconnect as you describe.

I've posted before, no one has an answer. Dell blames it on the use of XP, USR blames the Hyper threading processor and/or the 800mhz bus. 

You and I must be the only ones with the problem.

If you ever get an answer, please let me know.

Ron