Sound distortion on some music.

Hello. I hope you can help me because this problem is giving me A LOT of headaches V.V
first of all, im spanish so dont expect a great english.
Let's go;
The main problem is; i hear some distortion on some music tracks. Its a short distortion, like a "pop" that you hear when you're plugging a jack on the computer.
Its not problem of the music file, as it happens randomly and i cant repeat it, and it doesnt happen on the computer. It's not problem of the headphones, as it happens in the onboard loudspeaker, and with other speakers (i have no other headphones to try with). It's not problem of the music player, it happens with the stock one, and doubletwist ( i haven't tried more)
It's something releated to the firmware i think, as I downgraded back to JM1 and distortions dissapeared (at least i didn't hear any). Now i'm trying with JPP as it's the latest official update with Kies, and it still happens, but less than with JPY.

I'm REALLY frustrated, i've wasted hours trying to analyze and isolate the problem but I dont know why it's happenning and how to solve it :/

Ok, thank you, i've tried locking CPU speed at 400mhz with setCPU and it works. Now that we've located the problem, what can we do?
I'd have to open setCPU every time i want to listen to music?
:/