Hi,
Does your 3pin reg have bypass capacitors connected to its pins?
Have you checked your DC-DC converter?
Check its output DC volts and AC volts.
The capacitors on DC-DC converters, especially Electrolytic types, are working a lot harder than in a 50/60Hz application.
They eventually fail, usually high ESD and/or low capacity.
A circuit diagram would be helpful, please not a Fritzy image.
Thanks.. Tom... 
