Over the past few weeks I have been seeing quite a number of CPU spikes for various types of firewalls ranging from FG60B to 310B to 800. In every instance the "ipsengine" process was consuming all available CPU resources on the firewall. After consulting with Fortinet there appears to be an issue related to the current IPS Engine. Ask your SE and they may be able to provide you with a pre-release version of IPS Engine 1.165.
In the meantime if you run into this problem you can run the following command to restart the IPS Engine:
# diag test app ipsmonitor 99