Starting problem with Nissan 200SX

[Cpt Phil]

I have a Nissan 200SX which is unreliable when starting.  When I turn the key sometimes it clonks once and then starts and sometimes it takes several attempts.  If the car is put into gear and gently rocked it starts.  When the car is warm it may start first time or may be even worse.  There is no pattern to the problem.  The starter motor has been replaced. I took it to a garage and they could not find anything wrong with it!! Any suggestions would be gratefully received.