Isuzu Forums - Isuzu Enthusiasts Forum - 1995 trooper heater problem

Hi everyone!
I bought a 95 isuzu trooper this year from Arizona. I live in Northern Wisconsin and this is the first winter that the vehicle has seen. (so far I replaced the old worn out auto hubs with superwinch manual lock-outs. works great!)
I'm having trouble with the heater. I can start the trooper and let it idle for 15 minutes until the engine comes up to operating temp, turn the heater on, and only cool air comes out of the vents. I popped the hood and felt the hoses coming from the heater core and they are not hot either. As soon as I bring the rpms up, the hoses leading to the heater core warm up and heat starts coming out of the vents. When I let the engine come back to an idle, the hoses and heater cool off again. And the heat never really gets hot even on a longer trip, just warm. The motor comes all the way up to temp just fine.
I have replaced the thermostat, and the coolant. I've made sure that the coolant is up to level. I disconnected the heater hoses going to the core and flushed out the heater core and the engine separately with a water hose connection (no problems there). Put it all back together and have the same issue.
Not sure what to try next. Any suggestions? It's cold up here!