This can be caused by either a blockage in the line (VERY UNLIKELY), or a failing pump. Sometimes a pumps impeller will get plugged up with gunk and it will not push as much water through the system and thus your boiler over heats.
You may also want to clock the appliance to make sure it's not putting out more heat then it should.
You can tell if your pump is working by putting a screw driver to the pump and then to your ear. You should hear the motor humming away. Have someone kill power to the pump and start it back up again. You should hear it start and stop. You can also pull the pump and check the impeller for debris like I described above.
This may be a stupid question but are you sure all the valves are open in the system and it's not just something silly like that? Also check to make sure your zone valves are actually opening all the way.
I am no mod but you might want to post this in the boiler forum.