90 nissan d21
Have replaced therostat and water pump. Flushed heater core and it is clear. Hot water coming out of the top hose and cold water going in the bottom hose (radiator). Little bit of heat idling and no heat running down the road. What is wrong?

The thermostat may be faulty and is opening too soon, or perhaps the t-stat was installed upside-down and is free-flowing?
The heater core may allow a little heated coolant to flow but not enough. That year vehicle probably has some mile and the sediment may be too heavy to properly flush. Some of the heater core may be allowing just enough to make you think it's flowing good. Try removing the outlet hose and see if you get good flow with the engine running.