I had a similar problem what I did was drained the whole system than put tap water thru the whole sys. Darned than filled the hole system with a cleaning agent (i.e. house hold cleaner) let that sit over night darned that and then rinsed that out, then I took the heater core out and flushed it with a garden hose and put it all back together and worked grate. Sounds like a lot but the sludge that came out looked just like the ghost from ghost busters the little green thing.
