Just something to try maybe...
Assuming you have an overflow container installed, you might want to try a 4 lb. radiator cap until it STARTS to overheat again. When it starts to overheat, you should be able to see the condition of the coolant that's coming out into the overflow. Look for bubbles, discoloration, etc. With the low pressure cap, you should be able to CAREFULLY remove the cap to see if it's circulating also. I don't normally recommend it, but you've obviously got something strange going on here.
If you ever use radiator sealant....kiss your radiator goodbye. It contaminates the radiator and makes it useless. It will sporadically overheat just like you are describing.
Dont use the 180 t-stat. The 195 allows the water to remain in the radiator longer. The cooler t-stats tend to hang open all the time and water in the radiator doesnt have time to cool.
Dont run more than 30% antifreeze. Water cools much better. If I survive like this in a winter driven mustang in new england, everyone else should be fine too.
Dont have stuff in front of the radiator. I had a transmission cooler that was causing my car to overheat. I remove it and suddenly no problems. Maybe pull the AC condeser (?) away from the radiator temporarily.
The new cap might be defective. If the car is spitting out coolant after you shut it off, there is water boiling in one or both of the heads, which run much hotter than the average coolant temperature. A 14 psi cap will raise the boiling point by over 50 degrees over an open system. Even with straight water, the boiling point should be about 270. 215 seems a little cool to boil water at 14 psi
The cap also should allow coolant back into the system once the pressure in the radiator drops 3 psi below the atm, if the valve in the cap is bad, it will cause the lower radiator hose to collapse.