The critical value for the Damping is 48. Below 48 the particles stay on (as they should be) and above 48 they get instable.
http://www.darthfurby.com/genete/Other/ ... ility.anme

In the sample file I've move the particle layer around a curve and I've used those values to try to create a trail of particles.
I was very surprised when I inserted 60 on the Damping force and obtained a running group of particles.
-G