coolant system

[The Prez]

I need help... Whenever I drive the car (2006 2dr civic) on city streets the temp gauge goes up to optimal temp and the vents blow hot air. When I drive on the highway the temp gauge drops to 3 bars and if I turn the heat on it drops to 1 bar and the vent lets out luke warm air? I replaced the termostat and bled the air out of the system but it didn't help.
I don't know if this makes a difference but before this started happening the car overheated while sitting in traffic. Coolant was spewing out from the overflow and I was... (ahem )... smart enough to open the rad cap and release all the pressure and coolant (so I ended up replacing pretty much all the coolant).
Please help.

 

[JCCLARK]

Make sure your radiator fan is coming on when it gets hot.

 

[The Prez]

I was looking at that and I probably should have mentioned it but when I was bleeding the air out of the system the car was running for about half an hour and the fan never turned on. Also during that time I had the heat cranked to full and the temp display never got past 3 or 4 bars.

 

[JohnS.]

Not sure with the 8th gen's but with a 6th gen, I would recommend replacing the thermostat.