Heat blows cold at idle/rest, and proper hot temp when driving. What

Still seems like air in the heater core, very hard to get out. Can you access the 2 heater hoses going into the firewall, one is in and one is out, what I do is locate the out hose, figure out where the heater core metal pipe ends inside the rubber hose and where the clamp seals the hose, pull the hose past this point without totally removing it, drill a small hole in the metal pipe on top to bleed the air out when at higher engine speed but in Park, while filling the radiator at the same time to keep the water level up. once you feel the core is bled, push the rubber hose on and tighten the clamp, this should do it.