Heater not working properly

I have had to deal with this problem for the last 3 years! (Winters) When I'm driving or parked at operating temperature I can turn the heater on and it blows hot around 10 seconds or so then gets cold. Turn it off and back on again I get the same results. I have repeated this procedure more times than I would like to say. I get the same results almost all the time, with one exception, if I stop somewhere and I switch from defrost to vents before I shut the car off. Then get back in and start it, it has blown hot and stayed hot a couple of times. If I go to any other settings, it goes back to luck warm or cold again. I was just lucky those times, I think. (My opinion) Turn on and it's warm 10 seconds then goes cold. Turn off and back on and it's warm again 10 seconds then cold again. Bottom line it barely keeps the windshield defrosted but it won't keep anybody warm. I have changed the thermostat and checked the heater core hoses they seem to be hot. I have got the dash laying in the front seats of my car right now because I initially thought it was my heater core and bought a new one. On this car I still have to remove the whipper unit, A/C lines and more less the HVAC unit completely out. Is it a heater core issue or would it be a Blend door actuator issue?
PS: also changed the climate controller.