help it just stop's

To start off i'm not much of a motor guy, body, ect. not bad my son picked up a 73 duster 340 it start's up ok when warming it up then it will just shut down try to start it just turn's over won't start it's like there is no spark , wait 10 min. try it it start's wright up no problem take it on the road cruises ok but give it off the line it start's off like hell then shut's down dead wait a while it will start up again
what we have is a 340 with a older holly ( 70's) elc. fuel pump can't see
any #s 'blue in colour at gas tank line run's out side of the car protected to fuel pressure gage to 750 holly duel line
7 msd elc. ignition also what should the psi be at the fuel gage thank's for the help