OK, new IGBTs and some experiments. Please take a look and help us understand what's going on.
These are all one IGBT with a 12V battery source running a starter motor.
All have the same time scale and V scale except the last trace (V scale increased to show the driver signal, time scale the same).
C-E trace, no snubbers, increasing but low PWM, amps probably around 20
One snubber added.
Two snubbers added (though one had long leads)
same with stable PWM somewhere around 50%, current at ~35amps
The drive signal with 12V source and starter motor load.
The snubber. They came with the used IGBTs and we can't find any info on it.