I'll take a wild guess and say the alternator/voltage regulator is at fault. Check alternator's current and resistance at both cold and hot temp and see if they match. Just a voltage check won't be conclusive in your case
Another possibility is, if all previous mentioned is fine, then you have a bad connection or very tiny crack in a wire somewhere in the circuit (e.g. battery cable, alternator cable or a ground somewhere). When HOT cables/metals expand and that could explain why you are not getting the flickering when hot. A very tiny crack in the cabling/connection can produce this flickering crap.