Random freezes on VGN-NS21S (SD/USB related)

Hello everyone!

For a little more than a year I've owned a Sony Vaio VGN-NS21S, and ever since the beginning, a strange problem occurs:
While I'm using it, sometimes after hours, sometimes after only a few minutes, the computer freezes without any warning. At first, the active window doesn't respond to any input anymore, sometimes you can move it around, but you can't close it anymore. It isn't possible to open any more programs (not the task manager either), the cursor becomes the “rotating circle” (the waiting symbol). Copying or download progress stops, and the HD-access-light doesn't blink, but shines all the time. Even when I wait for hours, the computer doesn't “wake up anymore” and it's only possible to shut it off by pressing the power button for a few seconds.
The problem mostly occurs while I'm copying data from SD-cards or USB-flash drives to my hard disc (it's almost reproducible), but it also occurs while I'm working without using SD-cards or USB-sticks.

Sadly I don't have warranty anymore (I lost the receipt and the store can't find any data regarding the sale) and so I'm trying to fix the problem myself.

Tests I have already tried:
- the problem occurs with identical symptoms while running Windows 7 (32 bit) and Ubuntu Linux (32 bit and 64 bit versions)
- in the syslog of both systems there is no data regarding the crash– the entries stop when the first window freezes
- memtest86 doesn't find any mistakes, even when running for more than 5 times
- searching for bad blocks with fsck doesn't find any problems
- SMART gives optimal results about the health status of my hard disc
- the bios is the latest version, also all drivers for Windows are updated to the latest version
- copying (duplicating) data on the same hard disc (the internal one) doesn't lead to crashes, only copying from SD cards or USB drives

Does somebody maybe have an idea what I might have missed?
Thanks for any answer!
Yajua