Hmm, I haven’t had a problem like that before, so I’m not sure. I’ll point you in two directions though. You might try looking at "Responsive AA" in the material of the particle.
And also there’s the concept of “fireflies” with pixels that are too bright if you’re using raytracing. If you search in the console command bar you’ll find one related to “firefly suppression”. I think this is less likely to be your problem but could be.
I don’t think it’s strictly a sampling issue, since you wouldn’t be happy with it at runtime either, like you’re experiencing.