Particles flicker after a few seconds

I am working on a rain squall system, which consists of a cloud and rain falling below it. The cloud portion is fairly simple, but after a few hundred particles it begins flickering wildly.

I have tried disabling the rain, changing every sorting option I can find, and changing every material option I can find. Nothing stops the flickering once that particle limit is hit.

If you fly into it the cloud, the flickering will diminish until it stops; when you have less particles in view. So it seems entirely based on not being able to sort a given number of particles.

The particles are large, about 200 meters across, but changing these to very small made no difference. I am using subUVIndex, but removing that had no effect either. I am at a loss. I just cannot figure this out.