It looked as though reformatting fixed my 32 GB Sandisk reboot problem, but after 10 days or so, the reboot while idle returned on 2nd Jan-2012 at 9 AM :(
The only thing that just might tie in with this, is that my grandchildren were giving games, which reside on the SDcard a good run for their money yesterday evening.
I'll leave the card in and see how it develops. Perhaps the promised upgrade to ICS for Galaxy S2 will fix something... always the optimist!
Happy New Year by the way :)