Heat comes and goes....?
 

Great ever since I got all the work done on the MC the heat is been on and off, blower motor is working fine it's blowing at full capacity and the coolant system is been bleed over and over to release the air trapped but ever since my heat comes and goes, like for 20 seconds it work and then it's blows cool air for another 20 seconds then switches back to hot air. Is it the heater core saying its last good byes?

That sounds to me like a low coolant issue...or.. you have air still in the system... Did you bleed it with the motor off?
If you didnt.. then that's the problem it has an air pocket still in the system. That's causing the hot cold issue...
Also you may need to bleed it multiple times. Sometimes as many as 10 times. Be sure to open the bleeder in small increments. .. Being careful to not burn yourself..but be sure the vehicle is at operating temp..before you bleed the system.

The heater core leading to and from are hot as hell to the touch so are the radiator hoses. In idle (warming up) the car has heat the entire time it doesn't blow cool air unless I'm in motion.

I'll wait for a few minutes for the car to cool down but this is gonna be attempt number 50 bleeding the system

Going at attempt # 55 lol I'm determined to find a resolution to this issue

I know a few things that can cause what you describe.
- Tstat not opening properly
- leak in the cooling system, causing the air pocket (grabbing a radiator pressure tester can help diagnose that issue).
- Clog in the cooling system (but I would think you would have heat when you're RPMs are up and lose heat when you are stopped).
- Failed head gasket.

In the 3100/3400 cars, you can have a failed head gasket causing compression gases to to get into the cooling system and also over pressurize the cooling system (which is really bad long term).
What does your temp gauge read when your blower is blowing cold after you just had warm/hot air?
I just wrote a novel about what I know regarding failed head gaskets on the 3100/3400. The link is below:
https://montecarloforum.com/forum/ne...66/#post586956

I had a similar issue. A tech at my job put it up on a lift and noticed that I was leaking coolant from the water pump. I got a new water pump for $20 at Advanced Auto and had it put in, and I never had the problem again. Not to mention that the system was also bled about 5 times.

you didn't say what work you had done but sounds like you've got a heater control issue in the heater box itself. there is a door in the box that controls heat and a.c. parts of the box.sounds like the control of this door has broken or has come disengaged. that is why i was asking what kind of work. on early models this was all controled by vacume and allways gave trouble or the control leaver for the door would brake and the door would free float cold to hot like you expresed. i'd pull the glovebox and check the heater mech. or check the service manual at you auto zone or advance parts store.

@Rumpfox - The actuator for the flapper for the blend door is a good thought. But I'm going to say that is probably not it considering the OP is still bleeding air out of the cooling system. That should get resolved before ripping into the HVAC module.

Ok so I did a rad cap test & it doesn't leak any air, I did a compression test on all 6 cylinders and all are above 100 psi. Coolant not mixed with antifreeze, no white smoke! Odds of having a blown head gasket is slim to none right now.

What I got done as far as the cooling system goes is water pump, tstat, hoses, and new coolant.

I bleed the system many times with car on and off,

The temp stays at 180ish and air blows cool (not cold) and then switches back to hot air, the hoses leading to and from the heater core, one is very hot and the other one is just barely warm