screw starts then loads to point of belt slip

30 HP Joy stationary screw compressor. On start up it will run up to speed and then load up enough to slip the belts and almost stall the pump. New belt. New belt, proper tension, New oil, filter, oil air separator filter. proper oil level in sump. Lines are all clear - not plugged. Air end turns smoothly before and after episode. Ant insight would be greatly appreciated.