pug 106 starting problem!!!

i have a p reg peugeot 106 and i am having intermitant starting
problems with it. 90% of the time the car starts first time without
any problem, but, on occasions, once i have entered the 4 digit
security code and try to start the car i get no response. the ignition
lights come on as usual and all features of the car work as normal but
the engine wont even turn over. when this happens i have discovered i
am able to start the car with a bump start and it drives as normal.
the engine management light never comes on as a warning of any sort
and the problem is not consistant to any type of weather, different
types of driving or long or short journeys. i am a mechanic myself,
but just cant work this out. any input would be gratefully recieved.

 

Starter motor, feed to it from the ignition switch, earths on the
battery and the gearbox and chassis, and the ignition switch itself
(known problem)