What concerns/intrigues me is that @Linux.tar.gz doesn't see what is queueing the I/O. Take the following as an example from the same trace I cited earlier. You can see I/O being queued (and merged) prior to the (physical) I/O actually being issued.

I think we will all agree that a stock Slackware won't have this behavior (i just installed more programs on mine).
For now, i'm unable to explain what happens, even with your precious help. And it annoys me as i don't want to reinstall without understanding.
I have this .bash_history unexplained cleaning problem too. I'm close to think my machine is currently compromised. However, i ran chkrootdisk and rkhunter without detecting anything suspect. Plus i don't notice strange network traffic.
I'll need to think more and better .

Do you use kde ?

I have a laptop and when I am using konqueror and kde, laptop-mode just isn't very effective.
My disk goes to standby and whenever I switch tabs in konqueror or load a page, my disk has to wake up. I guess this has to do with cache management but this is a bit abnormal since ram should suffice (2GB).
If I run xfce, then I don't have this problem. This may be kde forcing sync but I can't be sure ; it could be worth trying with another desktop environment.

There's nothing to do with X.