serious lockups in 3.4.3 RC3

I installed RC3 a week ago or so on my machine. The install went smooth and it seemed to work OK. However, over the weekend I started to seriously use it and I have had 3 or 4 hard lockups until now. The only thing I did was upgrade udev, and install ksysv, apache, mysql and php. For the rest, the system is vanilla.

The lockups occured when working in firefox (opening the mepislovers page), konqueror (file manager) and ksysv. Symptoms: mouse movement works, but the system is completely unresponsive and requires a hard reset. I've had the occasional lockup with Mepis before, but that happened once every 6 months or so....

My setup:
AMD Athlon XP 1.7
ASUS Mobo (forget the type)
512 Mb memory
two sata disks
Nvidia geforce2 MX 400 (using Nvidia driver)
CD writer
DVD player

Nothing fancy and not too old fashioned either. If anyone has a clue?? Else I'll drop a note to dev-at-mepis.org.