Fortigate running 6.2.4 keeps switching to SSL decryption

I have a Fortigate 60E, running 6.2.4 (build 1112).  It works great most of the time. However occasionally it will start doing SSL decryption, and anyone hitting any SSL site through it will start getting traffic encrypted with the Fortigate's cert, not the public site's cert.  I can't figure out why the Fortigate just starts doing this apparently out-of-the-blue.  A reboot fixes the problem, but I can't explain it.  Anyone have any ideas where to start? Are there any known exploits that could be causing this to occur?  I believe I have the outside interface pretty well locked down, but of course it could something on the inside triggering it.