Just addin myself to the list - I'm having this exact same problem:
DS212j arrived this morning and has rebooted twice
within a matter of hours!
DSM version is 4.0-2219
Hard drives (that came pre-installed) are Seagate ST2000DM001 (2 off 2TB in RAID mode)
All I've done is let it synchronise the RAID, installed the Syslog server app and created some shared folders.
The web interface was open and I was copying a large number of files - both times it rebooted after half an hour or so.
As it appears (from other posts) to be a combination of high disk use and using the web interface so I've closed the web interface and am currently copying the files over to see if it happens again.
Then, I will apply the patch kindly supplied by HannibalK1ng and keep my fingers crossed it'll be rock solid from then on.
I'm really annoyed about this issue - I'd read many good things about Synology NAS drives and had no doubt about what to replace our old Buffalo Terastation with. I'm beginning to think it was a mistake - especially considering that I rang Buffalo tech support about the old drive, which is years out of warranty and got straight through to someone who told me how get it running well enough to get the data off it. I can't even get hold of the people who supplied the unit, let alone anyone at Synology.
I've put a support ticket in anyway, so will post back if anything comes of that.