Well, the problem is with the "sometimes." Flaky problems sometimes require flaky solutions, so you came to the right spot.
Restarting your machine might only be a placebo. It might be giving the real problem time to resolve itself.
You did not specify if you have dial-up access to an NT or W2K RRAS server at work or to the Internet.
If the problem occurs with the Internet, you might have an issue with LCP Header Extensions and software compression. Some ISPs have older terminal servers that don't recognize these PPP commands. Try disabling these two options. Open the Properties of the interface then select Networking | Settings. The fact that it happens occasionally might be due to the ISP having several banks of terminal servers, some of which are newer.
LCP Extensions isn't likely to be the problem if you are dialing into a private network. What kind of dial-up server are you using? If it is a third-party server like an Ascend box or a Cisco 5300 or a Shiva box, the box might be running out of resources. Is this a heavily trafficked dial-up service?
You might also be having spurious authentication problems. Are you using a separate authentication mechanism such as a RADIUS box or a Windows 2000/NT server configured to run IAS? Look at the network infrastructure at the dial-up site. Also, you might want to sniff the connection between the RADIUS box and the domain controller to see what's going on.
If none of this pans out, let's look at your machine. Are you using an internal or external modem? If external, do you have a history of weird dial-up connection errors? Are you using a reliable cable or one that you picked up at a computer show from a guy with lots of bandages on his fingertips from where the crimping tool slipped. You might also have an issue with the FIFO buffer in the RS232 port, although this is VERY unlikely. If the machine has a second COM port, it might be worth swapping the ports just to see.
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