1997 Kia Sophia hot water enters water reservor

Engine Cooling problem
1997 Kia Sophia 4 cyl Front Wheel Drive Automatic

I have.
-replaced the spark plugs with Bosch+2 prong
-replaced the spark plug cable
-changed the belts (only the visual ones)
-replaced the upper & lower radiador hoses
-serviced it for an oil change.

the car runs great (healthy feel and noise), until I have used it continueously (around 30-45 min-stuck in traffic). Then when I park after use. I hear water boiling. I open the hood and find that hot water enters the water reservor.

Im scratching my head over this one. While I wait for your ideas on what you would do, I will check for codes at Autozone.

The worried,
Johnny