Ok, so my Bonneville will run at 200 degrees consistently, but have no heat. Air comes out, just not cold. Then magically, sometimes, the air turns hot. Car takes forever to warm up when cold and forever to cool down after driven. When the heat doesnt work, the coolant lines were a little warm and then I raced the engine a few times and the lines warmed up a lot and the heat started working. Thermostat is new, maybe the water pump? The car isn't overheating though? Possibly a bad rad cap? Opinions are welcome, Thanks!

Sounds like the heater core....did you flush the coolant when you installed the thermostat? there maybe an air pocket in the cooling system...

I flushed the system throughly trying to get the dex-crap out and ran it a while open to let the air out. I even flushed just the heater core itself. It just seems that the coolant isn't circulating. The car doesn't overheat though. Does the car have a factory heater core bypass? Maybe thats the problem?

The heater core could be plugged. You could remove the lines and try to flush it.

Ok, flushed cooling system and made sure all air was out. Heat core was flushed, antifreeze added. The heat stayed ice cold until the car got up to about 205 degrees then the temp dropped to about 190 and the heat suddenly got really hot. Is my thermostat not opening soon enough to circulate warm air through the system? The car doesnt overheat, but it just seems the thermostat never opens sometimes. Let me know.

Stock temp is 190, so it all sounds about right.

yeah but mine runs constantly at 200-205ish. Rarely does it go down below 200. So the car can be at 205 with no heat. It almost seems like the thermostat isn't always opening like the heat rating on it is too high.

Pressure test it to see if there are any unseen/internal coolant leaks..My dads truck runs fine and at temp with no heat..It has a cracked head/block or a blow head gasket...I havent torn the motor down yet to see

Have you tried running the motor at 2500 rpm for 10-15 seconds AFTER the motor is completely warmed up after driving 10-15 minutes? Then repeat that 4 more times. That was the method for clearing air out of the heater core in the Factory Service Manual. I do it with the car slightly uphill hoping it helps the air work it'* way out of the heater core.

Then bleed the air at the thermostat housing.

Does the upper radiator hose get very warm after 10-15 minutes when the thermostat opens? It should be so hot you can't hold your hand on it more than few seconds.

Is the thermostat in correctly? Sorry to ask, but I think we should check. Is the thermostat in the groove in the middle of the rubber ring which is the seal?

Is the radiator full with the motor running above idle? Fill it then close the cap while someone idles the engine above normal idle.