I had the random reboots issue with the CSSU and pk48. I just enabled CSSU yesterday and after that I updated the pk from 47 to 48 and got the random reboots. I noticed the kernel-config settings reset themselves every time I lock the device, setting minimum CPU to 500, max to 750, governor to conservative and enabling VDD1 and VDD2. No matter which kernel-config profile I loaded it was always reverted.

I solved it by reinstalling nokia kernel and then installing pk48 again. No more reboots.

Hope this helps someone...