TwinDad:
My diagnosis is that despite the 10K pull-up on the schematic, the reset is floating and I need an external pull-up on the reset pin.Am I on the right track here? Is there something else I should consider?
The DTR pin on the serial interface should also be connected to reset on the Pro Mini via a capacitor, so when this serial lead is plugged into the the Pro Mini reset in pin (DTR) 'floats' in any case. In other words the serial lead is actually designed not to act as a reset line pullup.
You could just trace the components and check if the required components are there, but it sounds more like some sort of power problem.