Overheating Problem - Need Help!!!

I have 2002 Dodge Neon SE SOHC automatic. The car recently overheated on me on the highway this past week. I got it to a parts store and I changed the thermostat since I had plenty of fluid both in the rad and in the resevoir tank. I then drove for about 15 mins and the car started over heating again. I got it home and parked it.

Everytime it overheats the heater starts blowing cold air and then slowly but surely the temp guage sky rockets. Ocasionally I will see some white smoke come out the rear view mirror.

So I get home and open the hood and it looks like my resevoir tank is boiling and pushing fluid out the overflow, which falls right on the tranny and downpipe.....this would explain the smoke every so often.

So I had the water pump and timing belt changed since it had 115k miles on it anyways.

Brought the car home and same thing happened......took it back to the mechanic thinking maybe a heater core so he pressure tested the entire system for an hour and drove it for over an hour, no overheating at all. Brought it back to my house drove around for about 45 more mins and everything is fine. No leaks inside and no condensation, so i don't think a heater core either.

Then parked it for 30mins and went out again.....took about 12 mins for it to overheat once again. So I am back at square one.....new thermostat, new water pump and timing belt.

2 questions?
-Possible head gasket???
-IF so and i just get a new engine, what does an EGR port look like on the intake??

There are a few possible problems:
1)radiator cap is defective or not on correctly
2)not the proper mixture of coolant and DI water, should be 50/50
3)bad/clogged radiator

I know the problem most likely has been fixed, but I think I have an answer the transmission is making it overheat. All the cars do not have enough cooling when problems come up. I personally will not drive any car long with out fixing any cooling system problems and adding a trans cooler.