Hi, show us the DC-DC converter, what are you using for wiring and how are you holding the batteries.
Have you measured each cell when they are under load to check if they are all dropping in output or just one?
Sounds like they are not fully charged.
Can you measure the current being drawn from them when the fault occurs.
Also the current being drawn with the C cells.
A picture of your setup would help.
Tom...... 