i need a most plausible explanation why over the last million of years
the magnetic field of the earth turned around a couple of times as
proven by geologists. Thanks, Puzzled Peter

This may be of interest to you:

http://www.pulseplanet.com/archive/Sep04/3281.html

It is a meta-stable system and falls victim to chaos theory.

Best

The magnetic field is due to magnetohydrodynamics: fluid motion of
electrically charged media creating a magnetic field.  The fluid
motion in the Earth's case is thermal convection in the fluid outer
core.  This fluid motion is quite complex, and is influenced by heat
dissipation, composition and the Earth's rotation.

Background: 
http://www.nasa.gov/vision/earth/lookingatearth/29dec_magneticfield.html
http://istp.gsfc.nasa.gov/earthmag/dynamos2.htm

The Glatzmeier/Roberts model of this dynamo produced a reversal.  It's
the best model/theory I've found to explain the reversal.  Here's more
info on that:
http://es.ucsc.edu/~glatz/geodynamo.html

The sun's magnetic field also reverses itself, every 11 years.  This
is associated with the sunspot cycle.
http://science.nasa.gov/headlines/y2001/ast15feb_1.htm 

Hope this helps.