Nano crashes, won't reset unless reprogrammed

I've built a fuel injection system (gas injection, actually) for a model engine using a Nano. I bought the Nanos in bulk and they have the old bootloader.

The electronic ignition for the engine makes the Nano crash fairly frequently, but I'm working on that by adding decoupling capacitors, inductors, shielded cables, etc. This question is not about avoiding crashes in an electrically noisy environment - that's for another thread! :slight_smile:

When the Nano crashes it stops responding to the serial port or any I/O ports, and just sits there with the 'L' led flashing slowly - about once per second, roughly.

Now this is the weird part: it won't reset when I press the reset button, or when I power down and up again. Once it has got into that weird slow-flash state, even powering off won't fix it.

I discovered by accident that if I plug my USB lead into the Nano and reload the program it starts working just fine again. In summary, once it has crashed:

1/ Pressing the reset button doesn't fix it

2/ Cycling the power doesn't fix it

3/ Downloading the program again does fix it

What could be going on? Why does it start working again after reprogramming, but not after a reset or a power down?