Hi,
"A 6 mils trace" without thicknes, without length....so it's hard for us to imagine the whole proble.
"The trace gets very hot" What is "very hot" 60°C ? You won't stand that heat by touching it. Or 150°C, or 500°C?
"Very sporadic" does not mean much to us. We can't calculate with sporadic. If "sporadic" means that it's possible the the 100ms pulse comes several times within 10s ...then most probably the heat can't get spread enough and temperaure will rise.
How much? Impossible for us to answer.
What about other influences.
* ambient temperature variations
* air flow variations
* dirt
* humidity
* other heat sources nearby
* ...
So maybe your design fails within minutes, or maybe it never fails.
We can't know.
Klaus