i am having a problem with my 73 challenger shuting off after 15-20 minutes of running any ideas ?

 

what do you mean by shutting off?
does the car sputter, then die?
sounds like ignition to me
could it be the ignition box: when the car gets hot, it stops working.

when the car cools down after a while, does it start back up?

 

It's hard to diagnoise a problem with a car when there is hardly any information..

My diagnoisis to the information you gave would be vapor lock;

But it could be any amount of any other things. More info is needed..

 

Yep, more info needed. How do you know it's a SPARK problem, what have you done to trouble shoot?
Will it restart after it shuts down? Could be the ECU or ballast resister. You can put a jumper wire accrost the ballast and see if it restarts. The ECU needs to be WELL grounded to the firewall.
Take the coil wire out of the dist. and hold it 1/2inch from a ground (not thr carb) and see if you have spark when the problem occurs.