jerky particle pulses

I’m rendering a beam pulse using a particle system and halo materials to get the glow. Though it looked smooth in the preview view inside of blunder the glow around the particles seems to pulse and look jerky (it’s not a steady constant light and the particles seem to bunch up from time to time) as it moves along. I’ve attached three frames from the animation since I don’t have a youtube or vimeo account to post the actual animation.

It’s supposed to look like the long version in frame 292 (last frame) but it shrinks every few frames back down to the size in frame 290 (first frame).

I rendered the animation as png stills with 90% compression before rendering the movie. Not sure what’s causing this behavior but it makes the animation look terrible.