2008 GMC Envoy won't start up sometimes

my 2008 envoy wont start up sometimes,when I turn the key all lights work but no start up the shifter wont come out of park.if I keep turning the key back and forth for a while it will start right up and it has stalled on me while drivibg it a couple of times,we replaced the ignition and purge valve and we didn't have this problem for about 4 months now the problems are back.it never has left me stranded all I have to do is keep turning the key back and forth and eventully it will start
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