My car is still overheating

2000 Pontiac Grandam GT, V6 engine. 97,000 miles. My car overheats after about 20minutes of driving in stop and go traffic, 10 minutes highway. 2years ago I put some stop leak in my car for a leaking coolant somewhere. Got the water pump replaced. 2years later thick gluey residue where coolant go. Because of overheating went to tech, changed my water pump, thermostat, did flush, and changed Radiator $600 repair car still over heating, go to another tech, he says intake gaskets cracked which is causing water leakage big recall on this $800 repair. Got this done.

car still overheating take to GM, they dont know what causing problem no cracked head gasket intake manifold okay, Radiator fine etc. Feel may be clog somewhere but want $95 hr just to tell me something I told them already. I noticed when they changed radiator the thick gluey residue stuff was clogged up in.

My heat goes in and out. The coolant comes out of the overflow where the coolant goes in at like threre is a blockage somewhere in the lines. It bubbles up and overflows through the overflow tube. I hear noise in the car as I am driving like water trying to get through somewhere and when low coolant light come on heat blows cold air. Fans work can come on fine but car overheats all the way to the red. I cut off to let engine cool all the way down. Everytime I pull off in stop and go traffic after 15 min of driving I have to pull over and stop because engine over heats.

I feel that thick gluey stuff it is a brown orange color is clogged somewhere in the heater core and water is not passing through and this is why car keep overheating. Car can stay in park and run for hours and will not overheat but when you take out of park and drive 15 minutes it is too hot. I feel heater core or connections to heater core for water is clogged because the heat comes in and out it just sound like a bunch of water is trapped somehwhere. What do you think problem may be? Please Please help and respond asap. Thank you in advance.