2000 chrysler concorde overheats?

[Yulanda]

My car recently started to overheat. Changed reservoir cap, thermostat, water pump, timing belt, v-belts last week. Car ran fine for 6 days, started overheating again. Temp Gauge fluctuates when driving or idling and coolant bubbles in overflow tank. Heater was working fine but heat went cool on me for a few seconds yesterday. Thought I heard water or something moving around inside heater vents. Tried bleeding air through bleeder valve but still overheats. Fans seem to be working correctly. Have checked oil & antifreeze, no sign of head gasket problems. What have I missed? Could it possibly be a bad heater core? Would that cause car to overheat? Thanks