Any idea what causes this?
In no particular order
Improper tune, timing too slow Do NOT necessarily time car "by the sticker" which was for best smog, not best driveability
Generally, condition of ignition components, cap, rotor, wires, plugs
vacuum leaks, including bad brake booster and or hoses and fittings
problems with carb and especially idle circuit or even just incorrect idle mixture
generally poor engine condition, worn rings, valves
worn timing chain/ cam drive
In rare cases, problems with transmission, IE converter