1990 Geo Prizm Continually have to jump start car

Please please help!!!!!! I have replaced the battery, the starter and the alternator and my car refuses to start. There is no noise when I turn the key. It will work when someone gives me a jump. What else could be the problem?
It does this about every three to four weeks. Once it begins not to start, it continues to do this every time you try to turn the ignition on. Then it will run fine for a couple of weeks and then begins to do it all over again??????

Any help or suggestions would be greatly appreciated.

Thanks!