starting issue

guys, can you help me out with this starting issue -

My car is not wanting to start a lot of times when it gets hot - Sometimes I will hit the key and the starter will not engage (like it has hit a bad spot) If I short the solenoid with a screwdriver it will turn the starter over and then she will turn freely again. I have replaced the starter with a pretty expensive one from NAPA and still does the same thing
I do have headers and am wondering if they along with the heat is causing any of this?

Any thoughts??

PS - 360/727 68 barracuda