Engine Overheating Mystery

Car: 2000 Honda Civic EX, 1.6L 127 hp I4, 4-Speed Automatic Overdrive,

Problem: My engine is overheating. Whenever I drive in stop-and-go traffic for 30-45+ minutes my car starts to overheat -- in cold weather, hot weather, with the air conditioner on or off, with the air vent on or off (although, I’ve noticed that driving with the air vents on the blue/cool setting will make the car overheat faster than if I dive with the ac on).

Additional Info:
The temperature gauge will almost immediately return to near-normal levels upon turning my heater on. The car doesn't overheat with freeway driving.

My timing belt was just replaced. I have no blockages in the hoses. The hoses are new. The thermostat was also just replaced. The radiator has been flushed. The coolant mixture is balanced to the correct levels and does not fluctuate when my car overheats. My oil levels do fluctuate, however: when my car overheats, the oil levels drop. I don’t have an oil leak, nor is their any sign of water mixing with my oil (i.e. a murky gray film or color on the underside of the oil filler cap) nor does my car begin to smoke or let off steam when it overheats.

The fan seems to be working (when idling, the car will heat up, but the fan will turn on when the temp gauge climbs about to the mid point -- cooling the engine back to the normal temp. However, when I drive in stop/go traffic, the temp gauge climbs to the highest point, and will only drop once I turn on the heater on full blast), but my mechanic still thinks that my fan switch may be the problem. I just had the 100k tune-up.

Knowing absolutely nothing about cars, I asked my mechanic about the possibility of a blown, cracked, or otherwise damaged head gasket (which seems to be the consensus diagnoses in other forums dealing with similar issues) -- he scoffed and said that wasn't the problem. Hopefully, he’s right. That would be a huge kick in the pants.

Anyone have any idea why my car is overheating?

Thanks.

If your not losing coolant, and your level is always good, then it's not a head gasket causing your issues.

If your heater cools it down, then your water pump is probably working.

Sounds like a fan issue to me.

As far as losing oil when you overheat, you're probably burning it. Hotter it gets, the thinner the oil is gonna be and the easier it will pass by the rings.

my mechanic still thinks that my fan switch may be the problem. .
I think he is correct.

Has your mechanic cleaned the outside of the radiator? They can and do accumulate debris that limits the flow of air through them. Blowing compressed air through the radiator from the engine side will clean out the debris that has built up in the fins. It certainly can't hurt to give it a try!

Might be a plugged up radiator.
Even though it has been flushed, could still be clogged.