If the rad cap isn't holding pressure, the expanding coolant will drain into the reservoir first. That doesn't necessarily mean you'll see a leak. More importantly, the lack of pressure will lower the boiling point of the coolant/water mix, decreasing its latent heat absorption.

Sounds to me like the thermostat isn't opening fully, or you may have air trapped in there.

With the engine dead cold, remove the radiator cap and start the engine.
Let it idle until the coolant temp fan switch cycles at least once. Any air in the system will migrate to the open radiator; check on the car regularly to ensure it doesn't run low or belch coolant onto the paint.