I can recommend a few tests/ideas -
a) Run some diagnostic software on your harddrive to check for errors.
b) Get a new SATA cable
c) Run a memory testing program (although this sounds very unlike a memory issue....usually those = nonrecoverable lockups).
I personally had a bad ide cable about a year back that was causing issues like you are mentioning. It was very frustrating to diagnose, but a new cable did the trick. If you want to try something, while a lockup is occuring, jiggle the sata cable a bit and see if that unfreezes the problem.
If you want to do some generic 'testing', try finding VERY large files/folders on your drive and copy->paste. This will really run the drive thru some work. To really help pinpoint, pull everything from cpu except vidcard, hd, and 1 stick of memory (and cpu of course 🙂 ). Then run the harddrive tests.
Other testing would be to go to controlpanel->administrativetools->performance and see type of counters spike when the lockups occur.
Good luck....