This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
When setting a particle system to be active after deactivating the remaining particles still floating in the air from earlier will flicker. This is harder to reproduce when smooth frame rate is enabled. The flicker is more consistent if the camera is closer to the particle system and the frame rate is fixed.
The Particle System will have a slight flicker once reactivated this is noticeable on the particles left behind still floating in the air.
There is no flicker
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-37666 in the post.