An unexpected phenomenon observed in particle settling: oscillating falling spheres

We have observed the phenomenon of oscillating particle settling. A particle settling in borate cross-linked hydroxy propyl guar in a pipe with a relatively large particle/pipe diameter ratio regularly accelerates and decelerates while it is settling. The phenomenon can be described by a particle that settles slower and slower until at a certain point the particle suddenly accelerates as if it is settling in a Newtonian fluid and then start slowing down again. This cycle regularly repeats itself a number of times before the bottom of the pipe is reached. The phenomenon has been reproduced a number of times with different fluids.