spark, working, not working

[davehdicus]

my 1969 F-250 390cid. Runs good till it warms a little then no spark . wait an hour starts drive to store drive around town parkiy. go out next morn fires right up, idles a few min. than dies, check no spark. new coil, dist. parts. voltage reg, start relay. the only thing I havent changed is the alt. have a new one just havent tried it yet. nww wirs from ign. switch. , perplexed???