All loopers and some crossflow OMCs had the self-shutdown in case soemthing went wrong. It was very primitive, but it did work. And they had the VRO, which mixed the oil for you. These had a tendency to fail, and they would cause the motor to shutdown when there was no oil. You could have air in your oil line, causing the motor to slow down when it hits air, and get back up to rpms when it detects oil again.
Do you have an alarm or idiot lights? If you have an alarm, it should sound if it was something computer related, unless the buzzer is broken. If you have the idiot lights, they should tell you whats going wrong. (i.e. overheating, low oil, etc.)
Do you have a temp gauge? Does the temp shoot up when it slows down?