Spread the pins out from each other in the ecu, this causes the pins to be a bit more bound when you connect the harness plug. Also make sure none of the pins in the harness plug are loose or pushed back. Put dielectric grease in the plug before final connection.
Also so a good idea to wrap the power wires with electrical tape or shrink wrap under the on/off/start dial. If the battery box top becomes unclipped and arcs the leads onto the actual battery box, you will fry the harness.
These 2 fixes have taken care of 90% of the issues we have come across.