I've been having this pretty same problem since I got this laptop. I don't know if it's a software (driver) or hardware (drive, firmware) problem, but it's completely annoying. In Ubuntu Edgy with kernels 2.6.17 it just freezed and I had nothing to do but reboot the computer and I could not see kernel messages. The freeze was step by step: first music (amarok. alsa) stopped, then only I could move the mouse but it didn't answer to mouseover or mouseclick events, and finally (when I pressed a keyin the keyboard or after some seconds) it freezed completely and I had to hit the reboot button.

Now I've switched to Ubuntu Feisty and it doesn't freeze anymore, but it gets really slow and as you said the system only answers 1 out of every 15 seconds. But now that's better because I can still see dmesg, and I see this message repeated all the time:

[45395.837435] hdc: status timeout: status=0xd0 { Busy }
[45395.837441] ide: failed opcode was: unknown
[45395.837726] hdc: drive not ready for command

Same as you. Sometimes music plays but nothing else works (not even mouse) for ~14 seconds. It usually works like this: it freezes for a while. Then I move the mouse to konsole and click, and I wait 15-30 seconds for it to show. Then I start typing dmesg and I hit enter and it freezes for a while again. After 15 senconds dmesg letters are shown in konsole, and freezes one time more before showing dmesg output, and once more when showing it. After that, I execute halt and I got for coffee because as you can imagine, it takes a while also to shutdown. By the way, there was never any cdrom in the drive!

It seems to me that there's somekind of kernel process getting very busy with extreme priority abusing of it. Probably kernel scheduling has changed a bit in kernel 2.6.20 and now it doesn't let this process own the CPU for more than X seconds, but because of the high priority of the process, it gets back the CPU very fast everytime it can, for example when I hit a key in the keyboard and it produces an interrupt. But this is all speculation.

This is my kernel:
root@palobajo:~# uname -a
Linux palobajo 2.6.20-12-generic #2 SMP Wed Mar 21 20:55:46 UTC 2007 i686 GNU/Linux

Now what's funny is that when writting this message the error appeared again, but this time it was different. First there was a CDROM in the drive, but it was not being used. But the funny thing is that after the related synthoms appeared for about 5mins, it ejected the cdrom and got solved. After watching dmesg, I saw that it now only ejected the cdrom but also disabled DMA in the drive, and this is what solved the situation.

See: 
root@palobajo:~# dmesg | tail
[ 8613.842944] ide: failed opcode was: unknown
[ 8613.842952] hdc: drive not ready for command
[ 8618.896114] hdc: status timeout: status=0xd0 { Busy }
[ 8618.896121] ide: failed opcode was: unknown
[ 8618.896129] hdc: drive not ready for command
[ 8623.953288] hdc: status timeout: status=0xd0 { Busy }
[ 8623.953295] ide: failed opcode was: unknown
[ 8623.953306] hdc: DMA disabled
[ 8623.953344] hdc: drive not ready for command
[ 8624.004671] hdc: ATAPI reset complete

As I don't use the drive that much, I think I'm gonna disable DMA and see if the problem doesn't appear anymore. To disable DMA, I've added this line to /etc/hdparm.conf:

/dev/hdc {
       dma = off
}

By thw way, this is my drive:  hdc: TSSTcorpCD/DVDW TS-L632D, ATAPI CD/DVD-ROM drive. And the Laptop is a Samsung R40. The attached file is today's dmesg output, which is the only one which presented an (automagic!) solution to the problem.