Who is advising you on this?
You are not clear if it's a problem with the engine and ignition - engine fails to fire - or - a problem with the starter motor failing to turn the engine over.
Given that you've had the starter motor changed and still have the same problem then it's probably more to do with the starter not working than the engine not working.
Whoever suggested to change the starter was taking advantage of you - similarly (even more so) the key.
A low current wire runs from the ignition switch to the very high current relay of the starter motor - as you turn the switch to the start position the relay is activated and the starter motor turns over.
If the wire is making a bad contact or there's a bad earth then the relay (solenoid) will not operate and the starter will not turn - if the starter motor itself were broken you would hear the click of the solenoid operating as you turn the switch - you don't usually hear it over the noise of the engine turning over.
The symptoms you describe sound as if the starter solenoid relay is intermittently not operating for some reason - I would suspect a poor connection in the control wire circuit.
This is pure guesswork - I know nothing about cars.
Good luck...
Claire Vouyant
September 2014
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