thought this was worth a post. my '95 EV blew out the heater core twice and the AT cooler, all within two years. My mechanic decided to pressure test the coolant reservoir cap and found it was faulty. These blowouts seemed to have occurred after going over mountain passes with the engine (and coolant pressure) heating up. replaced the cap and haven't had a problem. Having to add coolant after a long hot ride lets me know the cap is releasing pressure. Now I have the cap pressure tested when ever I have work done.
If you have to top up coolant because it is being vented out past the cap... that would indicate a head gasket issue to me.
If coolant is between min & max level, there should be no need for coolant to be venting out the cap... unless there is a problem (couldn't before because the cap was not venting... causing components to blow).
Have them test for combustion gasses in the cooling system.