Hi,
I have experienced the same thing, multiple reboots in one day, sometimes just froze and I had to manually reinsert the battery to make it restart. I think you won't like what I'm gonna say but here it is:
I sent my phone to have it fixed during the warranty period. The first time they replaced the whole main board. In 2 months, problem reappeared. This time, they replaced some cable. Guess what - the bloody thing rebooted within 30 minutes after I got it back. Third time they just updated firmware. After two weeks, same thing. So after some struggle I got a new phone - a different one.
So I'm sorry to say that I don't have any easy solution for you, you might have to go through this, too.
As to the SD card, I have 16gb sd card, too, and it worked perfectly with all the phones I had borrowed during the warranty repairs of my phone. So that probably isn't the problem.
Good luck!