Chrysler Repair: Dodge neon flash heating and coolant accumulation at the reservoir, head gasket problem, upper radiator hose

Question
I own a dodge neon 2.0 DOHC AT, with 125000 miles. Yesterday I experienced a very quick temp raise after about 10-15 min driving, the audible alarm went off for about 20 secs because the temperature gauge was at its highest. I reduced speed from 65 to 50 for about 2 miles and the gauge dropped very quickly to normal temperature. After that I cruised in the freeway at 60 mph and the temp gauge did not raise for at about 40 min.

Today I let the car warm up in the parking lot (about 10 min), made sure the heating was blowing warm air and went for a city ride for about just one mile. I again saw the gauge jump very quickly (although this time did not reach the highest temp and the alarm did not came off) and go down to normal also very fast (8-10 secs). I drove to the parking lot again, let the car cool down and check levels. Oil and coolant were nice and clean so I am (initially) discarding a head gasket problem. What I did not expect was a lot of coolant in the reservoir (way above the full level), and when I opened the pressure cap I notice the cap was wet from apparently clean coolant running through the upper radiator hose but I could see no coolant at all in that hose.

I would appreciate any help to diagnose this

Answer
Hi Carlos,
I would check the reservoir after having driven the car and allowing it to cool down all the way. Note the level in the reservoir after you turn off the engine, then check later to see if it has dropped after everything cools down. If not, then there is a leak of some sort because the reverse flow from the reservoir to the engine's cooling systems depends upon a vacuum forming there in, and a leak somewhere will circumvent the forming of a vacuum. It could be the radiator, a hose fitting, the radiator cap,  or a head gasket. So don't discard any of those. Listen for any bubbling sounds coming from the engine during warm up which would be caused by exhaust gas getting into the system due to a head gasket leak. It would also be noted as a too early pressurization of the system, or bubbling into the reservoir. If none of that appears, then look for a regular leak around the edge of the radiator, or at hose fittings which might be preventing the vacuum from forming when you turn off the engine and let it cool.
A radiator shop can test for exhaust gas in the cooling system by putting a sensor at the radiator cap position.
Those are my ideas for what to look for.
Roland