Overheating I have Idea what else it could be?

If you have changed all of those but it still overheats check that the water pump is circulating coolant. While not common I have seen the impellers fail on them. When you say it overheats do you mean it gets hot enough that it boils over or is it just hot on the gauges?

The heater core isn't really part of the cooling system in that respect. The only time it will help is if you have air blowing through it that can carry the heat away and that only happens if you have the heater turned on, so mid summer 100 degrees out you are not likely to want hot air pouring into the cabin so the core isn't really there for cooling the engine. Did you change out all of these parts because it was getting hot or did you change them out and then have an issue? The coolant coming out the reservoir cap sounds like it was over full. Are the cooling fans turning on?
The normal filling method on these is to start with a cold engine. Fill the system until it reaches the cold line. Then start the engine and rev it a couple times and let it warm up. Now shut it off. Once cool you top it back up to cold and repeat the process until it doesn't take any extra coolant when cold. No other steps needed.
Your description sounds like it has air in it.

A leak won't let air in unless it is above the level of the coolant. Plus, as soon as the system pressurized a leak would show up by leaking more. That it doesn't overheat every time, and the coolant was staying in the correct level, suggests either a bad thermostat that is sticking or the fans are not coming on and the coolant is overheating. However, the cooling fans would be noticeable. What was the vehicle doing when it got hot? Just driving or?

Simple test for the fans, just unplug the coolant temperature sensor. That should trigger them on.

The intermittent part of it is what doesn't make sense. I would expect it to overheat every time, not just once or twice. Try this, grab an IR thermometer or a thermal camera if you can find one as a loaner. Use that to inspect the system when it gets hot. See if you can find something like an area that isn't moving coolant. If the hoses are getting hot and the rad is evenly heating then check the actual temperature and see if the fans come on starting at 202 degrees, then to high speed when it hits 220 degrees.

Okay, the dash gauge isn't very accurate, you need to see what the actual coolant temperature is. With coolant in them they run well over the boiling temperature of plain water.
Overheating normally would be in a red area on a gauge or when it actually starts dumping coolant. I would measure the actual temperatures it's running at.