Buick(olds) 403/th400 help?!

I'm running a 1978 Buick(olds) 403/th400 running gear in my car.

Problem is this... when it gets I first start my car.. for 20 mins or so it runs great, but then....

But then it acts as if it running out of gas.. Or not down shifting?? Weird! It stalls after a few minutes & will not start back up for 15 minutes. Acts as if the battery is dead. Makes for a long drive!

Any ideas? Thanks in advance!

 

403 is a olds.....not to be confused with a buick

 

Are you getting spark when it won't start Does it still crank normally when it won't start? If no spark (I am guessing you are running HEI) then test your ignition module , it may be overheating. I have also seen the pickup coil fail and cause similar issues. Not the coil, but the pickup coil that is connected directlt to the ignition module. Be sure to test the coil when it will not start or it may test fine. To test the PU coil unplug from module and 1st test ohms between the two connectors, should be 500-1500 ohms. 2nd test between each leg and ground, it should read infinite or your coil is shorted to ground. Hope this helps.