CAT 3512B generator, erratic speed

In the process of commissioning a previously used generator the speed would get erratic after about an hour of run time. Checking the data log the ECU was seeing the throttle command start to spike and eventually drop off. Checking that throttle PWM signal at the ECU connector with a Fluke meter the signal was steady, no spikes or drops. Turned out to be a loose ground for the speed brick coming off the ground rail, even though when checking the voltage to the speed brick it never changed. Once the loose ground was found I could recreate the issue. Tightened the loose ground and problem solved.