Virus software
I had this same problem; every time I'd start to play a (bard) song, or open a bag in inventory, there would be a subsecond freeze.
I found that if I turned off "document" scanning on my virus checker (I use AVG), the problem went away. My guess is that every time you perform one of these actions, EQ is accessing a file from the hard disk, and the antivirus software is kicking in, and you get a freeze until it is finished scanning.
I only had to turn off "document" scanning, not executable or email scanning, so as long as you're not downloading and using stuff from unknown sites while you're playing, I'd think it would be pretty safe.
Hope this helps!
Gui