I'm fairly new to cars, especially my 04 Z, and don't exactly have anyone to turn to when it comes to repair questions like this, so I may have an answer right in front of me that's plain as day.

My Z has had no overheating issues at all, through traffic as well as on the freeway, that is until one night I took it for a spin at 3 am. I turned on the defrosters and everything seemed fine, the next day I took it out to get some food and I was completely overheating, with coolant spewing out of the coolant reservoir. After not driving the car for a while I bled the system (although I'm unsure of how well of a job I did), waited a day, and the next day drove it for over an hour without going over the center mark on my gauge. Now recently there was a fairly cold night and I turned on my heater in my Z, albeit for only 10 minutes at most, and drove home with no issues. The next day I begin to see a spike in the temperature again while on a small drive in the morning which only goes down as I press on the accelerator. Since the two events occurred after the only times I have ever used my heater since I recently acquired the car, I suspect there's an issue with the coolant system connected with the heater core if not the heater core itself. I also suspect an issue with the thermostat not opening all the way or my radiator being clogged and requiring more pressure from my coolant system.

Again I am fairly new to cars and any help would be appreciated. Thanks in advance.