SQL Server service shutdown due to 'bad?' SSL cert
There are applications out there that change the ACLs on the machine keyset container folder. When this occurs, legitimate keyset users loose access to these keysets. See KB 278381: Default permissions for the MachineKeys folders for the correct permission set.

If you want absolute confirmation, see BUG: You cannot enable encryption by using a certificate when SQL Server 2005 is running under the Network Service account for a 'workaround' (it involves compiling a C++ app that will reset the ACLs properly...). But you can simply set the ACLs according to mentioned KB278381 and see if the problem dissapear.

More related KBs:


Peer-to-Peer Framework APIs return a "PEER_E_NO_KEY_ACCESS" error message
How to troubleshoot the Data Protection API (DPAPI)
Error message when you use Service Broker or database mirroring to connect to an instance of SQL Server 2005: "Connection handshake failed"


I am yet to find which application changes the ACLs. My money is on some antivirus apps. Don't ask me how I know about all this random pieces of disparate infos...