If you have tried increasing your buffer size and made sure you have the proper bandwidth selected, then most likely this is caused by you having either outdated hardware (a Pentium or lower), an outdated operating system (Win 3.1), an outdated browser (Netscape 3, IE 3), or not enough memory, video RAM, etc.

The Windows Media codecs are optimized for Pentium III and run fine on Pentium II, but on a Pentium or lower, they choke completely. Also you need a decent amount of system resources, RAM, VRAM, etc., to get good results.