Heater not working

OK, I'm pretty mechanically inclined, but this one is throwing me. I have an 05 grand Prix GT with a 3.8 non S/C. No heat from the vents today. Temp gauge is going up, and stays up while driving which I would think would indicate the thermostat isn't stuck open. The lower hose is kinda cool, upper hose is hot, and both heater hoses are hot which I think would indicate not a heater core. However, I can squeeze the upper and lower hoses, the system isn't building up pressure, but I see no leaks. Any advice would be welcome.

Heater problem
2005 Pontiac Grand Prix 6 cyl Two Wheel Drive Automatic 36000 miles

I get cold air blowing from the vents. When I checked the hoses the top radiator hose is somewhat hot; the bottom hose is not so hot. My antifreeze level is good. What would be the cause of no heat?

We have a 2005 Pontiac Grand Prix, V6 that is no longer giving heat. We have filled the coolant and changed the thermostat. Both helped initially but after turning the car off or letting it idle, it stopped blowing warm air. The blower is working. It seems that after changing the thermostat yesterday that it blows warmer air when the RPM's are high but as soon as it downshifts, the warm air stops and cold air resumes.

Please let me know if there is other information I can give that would be helpful. Thanks very much and Merry Christmas.
Trisha