Overheating - Out of ideas... :S

All, my Altima's temperature has been rising above normal for some time now... Have gone through a few steps already, but still can not fix the problem. Can you help me with potential points to look at? The Altima is an '08 2-door, S version with the 2.5 engine.

Note: (I have never let the temperature reach the 'H'), always shut it off before too close to it.

The problem started several months ago, very mild symptoms... Whenever I drove for a significant time >20 minutes, and stopped for a discrete time - i.e. McDonalds drive-thru, the engine started heating above normal. As soon as I started driving and got to 30mph the temperature went back to normal. I could even place the car in neutral, turned-off the A/C and the temperature would regulate eventually. Since I drive mostly on high-way the problem hardly showed-up. With a just-in-case mentality, got a coolant flush done at the dealer. Also asked them to check the problem, did not find anything wrong, they could not get the temperature to rise above normal.

Months after, the problem start showing up more often and symptoms morphed.

As of now, I can not drive the car even on highway, after 10 minutes of driving the temperature starts rising. The only way I can keep the car in an Ok range is by driving at 50mph and blowing the heater at max.

I have now taken it to a local shop, they still can not figure out the problem, and while they 'dig' for it. I am starting to feel the bill pilling up.

Here are a few things that I have done to eliminates potential causes:

Fans - Both working
Thermostats - Both changed (I did not even know this car had two...)
Flushed twice now
Radiator cap changed
Radiator pressure checked Ok for leaks
Water Pump Ok
Have not noticed coolant lost on the reservoir tank and floor is clear of coolant
Have not noticed any trace of coolant around head
When temperature has risen and A/C was on air starts coming out warm

Any ideas up to this point?

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Today I did and experiment to try to dig even further into the problem. Not sure what the car was telling me though.

Set-Up:
- We removed the radiator cap
- Placed a thermometer on the radiator
- AC was off, coolant level in radiator was at max

The engine was started and let idle for the entire experiment.

Things noticed:
- Coolant temperature (measured at top of the radiator) never went above 200F
- Out of the two fans on the radiator, one expelled hot air, the second one expelled cold air (picture below)
- Saw what seemed to be bubbles, not sure if they came from outside-in when 'raising' (video below)
- Temperature start rising above normal after 30 minutes
- Never saw a 'flow' of coolant through inlet... although the inlet on this radiator is not exactly above the coils (see video)



http://www.youtube.com/watch?v=hlIjZgo2ySo&feature=youtu.be

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Thanks in advance for any help!