On 9th December I installed the old Kingston 8 GB SDcard from my Nexus One in the GS2, formatted it, and ran a MyBackup Pro backup to it. Since then it has been working fine, no reboots, tho' it is still early days.
I'm expecting a pair of 16 GB Class 6 SanDisks to arrive from Dustin in the next day or so. I'll replace the Kingston with one of these and continue the test for up to a week before replacing the original 32 GB Class 4 SanDisk, reformatting it, and seeing if I have more reboots with the latter.