Why do my online streaming videos at www.hulu.com freeze up and go in slow-mo while the volume is unaffected?

by John
(Philadelphia, PA USA)

It's not a buffering thing but the video starts out fine and then after a minute or so, the picture freezes and proceeds in slow motion as if it were skipping every few frames of film; yet, the volume is fine. My ISP is Verizon and they can't seem to help me.