sbc 400 stalling out need HELP

[southern gator]

chevy 1976 truck. Im having a heck of a time.
Im stumped on this. idles fine at start up, drive it about 3 mins down the road and if u let off the gas at a stop light it will stall out. Start it back up with gas pedal down, it will stay running if u hold the gas pedal down. other wise it will stall out. sometimes it will drive ok and then start to spit and kick. Also if u let it set and idle for say 10 mins, all the sudden the motor will hesitate and shake then dead.
Fuel tank is good/clean, lines are clear, new mech. fuel pump, new fuel filter, new carb. and mounting gaskest. Cant find any vacuum leaks. replaced all rubber hoses. I dont think its vapor locking cause if i open the gas cap when it acts up nothing changes. And i got 10 psi of fuel pressure all the time no change.
ANY HELP GUYS??