why are previews stopping after 3 seconds

I'm having the same problem. Also, after buying a music track my iTunes will not play it claiming that I have to authorize my computer... but it plays all other tracks I have previously bought. But it plays on my iPhone when I download it through the iTunes app.

Close your iTunes,

Go to command Prompt -

(Win 7/Vista) - START/ALL PROGRAMS/ACCESSORIES, right mouse click "Command Prompt", choose "Run as Administrator".

(Win XP SP2 n above) - START/ALL PROGRAMS/ACCESSORIES/Command Prompt

In the "Command Prompt" screen, type in

ipconfig /flushdns

Hit "ENTER" key

Restart your computer.

Now try your preview music again.