I have one STmini that was flakey with tc or without. The rba pos pin was barely touching the base pos pin when screwed in. I found it by looking through the largest airflow slots in the base. When I tightened the rba down I could see daylight on one edge of the junction so I knew it wasn't compressing. My other two STminis compressed together fully before the rba even tightened up all the way.
So I was getting res fluctuations from that one STmini but never popped a coil. Just radical changes in tc temp and sometimes my mod jumped out of tc.
To fix it I tapped on the bottom of the base 510 pos pin to make it move upwards before installing the rba. Installing and tightening the rba then pushes the pin back down and makes a better connection.
Mods with weak 510 connector springs can make the problem worse than ones with strong springs.
If this is what's happening with your STmini your mod may be seeing intermittent higher res and therefore applying intermittent higher voltage which may be breaking your coil.
Awesome, thanks @cigatron for the tip. I might try you're suggestion and if that works, as a more permanent solution put a bit of silver solder on the top of the 510 pin. Will let you know how it works out!
Thamks again.