Once you change the tank and cap report back. It is possible the cap is not holding pressure properly and letting coolant escape. That would allow for the loss and the lower pressure could be why it gets hot.
If you can get a cooling system pressure tester you could pump it up to the rating on the cap and see if the pressure drops. If it does not then the cap is the likely problem. If it does not hold pressure then you will need to check the heater core and see if it is leaking. They can leak and the only sign is a damp carpet or coolant coming out the HVAC case drain under the car.
Please report back once the new tank/cap are in.
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Thursday, September 14th, 2017 AT 7:17 PM