Peugeot 206 peugeot 206 wont start

I have a 2000 peugeot 206,1.4 lx, i dont have the car long but it wasnt long before it that at random times it would be weak to start and then it would start at all until bumped, it would then start no trouble the next 20 times until you had to bump again, at this point i thought the problem had to be the starter but when you turn the key you can hear the starter is working, i had the battery and alternator checked and both seem fine, now the car will not start at all without a bump so now its surely the starter but im thinking it could also be the ignition because when you start the car the car keeps screeching and you have to push the key back to stop the annoying sound, it makes the same sound as when you try to start a car thats already running.So im thinking, starter ignition or maybe ecu or electrical, the car is not really worth fixing unless im sure i know whats wrong