if anyone can help me plz do.

i have a 63 valiant wagon with a 73 318 out of a dart.
recently it has been running great but ounce in awhile 4 out of 5 starts it starts normal then putters down then i start it again and it starts fine but ill have to rev it over 1k to get its idle right when it does that.
anyone know what i could blook into, replaced battery/cables and starter those should be fine gas is good run gas treatment all tyhe time idk what it is.