Hypex UCD400 Amp Module Power Supply Pumping

[Normanguy55]

Has anyone experienced a power supply bus pumping on this module? I am using this module to drive my subwoofers. When it is connect to a 4 ohm load the -58.0 volt supply pumps up to -71.0 volts and the amp shuts down. When I connect an 8 ohm load the amp runs normally. I have changed several parts in the driver circuit (transistors, capacitors) but nothing helps. I am running a Half Bridge power supply with 10,000 uF capacitors. The amp used to perform flawlessly, until recently. I would appreciate anyone's help. Thanks.