2001 Impala Overheating

Our 2001 Chevy Impala, 110,000 miles 3.4L is having cooling issues. First it was leaking coolant, we brought it to a friends shop and he suggested putting stop leak in it. It's no longer leaking but it's going somewhere. We have to keep refilling it on a daily basis, the system was air locked but my husband found the bleeders so we took care of that. But it's still running hot. We've also replaced one of the temp. Sensors (apparently there's 2 but we don't know where the other is located) I was convinced it was the thermostat because it seemed like the temp. Would go up and not come back down, so we replaced that. Still, things seemed to be having trouble circulating.
Last weekend my husband took off all the hoses, heater core etc. And flushed EVERYTHING out (we thought maybe the stop leak plugged something up) and it's still doing it. You start the car, it runs ok for a very short while then it starts heating up and won't come back down. If we can get it to circulate heat then it's fine but half the time we have no heat. We're just at a loss. Thinking that it could be an intake gasket or head gasket (although the oil looks fine, not like a milkshake) maybe the other temp. Sensor. HELP!