That's a bit weird at the same time because it has this Voltage regulator in between. Maybe i'm missing something here, maybe it miss one capacitor or two here and there on the board to filter some high frequency noise coming from your Switching psu.. One thing i can say, is that i added a 10uF tantalum on each DIP socket too.

You could still use a AC psu, as this board have it's own diode bridge.
Oh well, at least it works