I also have a tune-up suggestion this time (just to make it clear: definitely NOT a solution for the lagging, just another tweak to make it a little less obvious).
Defragmentation of the affected harddrive has been suggested before, and lots of tune-up utilities will do this for you (don't just use the Windows defragger, it's slow and inefficient, any third-party defrag application is preferable).
But o&o Defrag Pro 8.0
has an option that is particularly interesting for us 'laggers': in addition to defragmentation it has the ability to sort all the files in a certain way, 1) according to name i.e. alphabetically, 2) according to when they were last accessed, or 3) according to when they were last modified.
Every Amilo 3438 user has different habits, of course, and each would have to decide which method is most useful themselves. Myself, I am affected by the lag infrequently when playing video and audio, which is why I opt for "sorting by name": the idea is to put all my big video and audio files at the very top of my file structure (in a folder that begins with an "a" or even an underscore, for example) which means that o&o Defrag will place these files at the very beginning of my disc where access time is fastest (at the end of the disc, access is slowest).
This should put my hard drive in a condition that is better than what I'd get from a "regular" defragmention, possibly with less lag. I am still testing, however, to see how it goes.