cooling system back pressure, heater not working

heyy, I have a 99 grand am GT and for some reason when you leave it running long enough it will shoot all of my anti-freeze out of the over flow. I've done a full flush, changed the thermostat, and taken the rad out to clean it. Even though i've done all this there still seems to be back pressure.

Also my heater doesnt work while all this is going on. When i clamped the #2 hose off in the picture below, all of the problems seem to go away and coolant starts to flow through the heating tubes again.

http://i42.photobucket.com/albums/e301/murfur/03Engine-Front-Centerr1.jpg

any suggestions would be greatly appreciated, i'd rather not have to drive around with vice grips on that hose hahah

make sure you have enough coolant and bleed air from the screw by your number 1 on the picture.

Make sure that your cooling system is coming up to operating pressure. When cold, squeeze radiator hose to see how it feels with no pressure. Start it up and let it run 15-20 minutes, till coolant temp is ~195deg. (stock thermostat temp). Squeeze the radiator hose, should be under pressure. No pressure = leak somewhere, and your coolant will boil out (coolant boils at lower temp when not under pressure). Usually it's the coolant reservior cap that leaks, its supposed to hold up to 15psi. You can get a replacement ~$5 at parts store. Sometimes it's the reservoir itself that won't hold the pressure.
Hope this helps.

thats very helpful thank you, i think i'm going to buy a new resevoir tank because there is a small leak in the side. could it be the intake gaskets? i've heard a lot of stories about that being the problem with grand am's

I have a little pet theory (no proof) that it's the other way around: that on a lot of these 3.4L v6's there's a coolant-pressure leak (either the cap, reservoir, hose, water pump, etc.... all common) that when it happens causes the system to loose presssure, loose coolant, get too hot in bad places, and result in/cause the LIM gasket problem to happen, when it might have been postponed or never happen if the system hadn't leaked. The LIM gaskets are always just waiting to go (re the critical concern about under/over-torqueing the bolts), and the multiple weak points in the cooling system are the triggers.
Speaking of weak points, are you using Dexcool for coolant? I flushed mine out real good and switched to prestone green stuff. Dexcool will get sludgy if a little air is left in after changing the fluid. It will also create sludge if it's mixed with green coolant, so if you switch you have to flush it out pretty good. Sludge can block passageways, cause hot spots, lead to LIM gasket failure. See http://www.automotiveforums.com/vbulletin/showthread.php?t=718130
for coolant-bleed procedure.
And i hate to mention it but: when you put the new reservoir on, immediately on start up check to see if the radiator hoses are under a lot of pressure. Lots of pressure before engine has time to warm up indicates a head gasket leak, which could certainly pop the reservoir in half.