if you couldn't find the answer yet.. try this..
you need to replace the TIME input node to the particle with an animation curve. I think if you just disconect the time in hypershade, then you can keyframe the time value on the particle object itself...
if you key 1 at frame one, and 100 at frame 100 - you will have a normal speed animation..
to get your effect, you need to key time to stop... for example:
frame 1 - key 1
frame 50 - key 50 (simulation runs normal speed till frame 50)
frame 150 - key 50 (simulation is frozen till 150- keeping frame 50)
frame 200 - key 100 (simlation continues at normal rate)
you can gradually stop it, or even reverse it with the right time curve.
The way I do it is to perfect the paticle animation in real time (ie make sure it looks ok with no stops or time effects), and write that particle data to a particle disk cache.
Then use the animated timeline on the cached particle data. It's a lot easier to manage this way.
hope this helps...
