Does it happen with bouncing the impliment? It could be that the increased cylinder pressure when the impliment pulls some "G"s is causing the hydraulics to overpressurize and the relief opens and vents fluid to protect the structure. If you have a draft control, it then adds fluid from the pump to reposition the
3PH after it drops from the lost fluid. If it dosn't do it while stationary, that would be my guess. If it hasn't done it in the past with this same impliment and under the EXACT same travel conditions, It could be that the safety relief adjustment has shifted, or something is causing it to relieve at a lower pressure.
What is the rated lift force on the
3PH? What is the weight of the impliment?