If this is a dial-up modem that came with your Windows 98 machine, it probably needs replacement. Dial-up modems can go bad just like all other equipment..I've had a few die on me after running great for years..

If this is a broadband modem, please tell us how it's connected and the brand and model number?

In addition, please tell us the EXACT error message you're seeing and when you see it.

Hope this helps and let us know more.