LM2678 Switching Regulator Voltage Dropout Under Load and extreme Input Distortion
The LM2678 operates at approximately 260 kHz and this means that you must choose an inductor with a lot of care and attention to detail. The inductor you have chosen is \$\color{red}{\text{not suitable}}\$ from the information that is available. Having dug a little deeper it seems that its core is iron dust/powder and this is likely to have significant eddy current losses at 260 kHz. What mouser says about this 47 uH: -



Highlighted in red boxes are the things that sounds alarms with me.

I'm not ruling out anything else but the inductor is a poor choice.