Low coolant level and/or air in the system?
Sounds like its only just pushing the coolant round the system when the engine is doing decent revs, which would only be noticeable at prolonged higher revs, such as 60/70 on a suitable road.
Maybe thermostat is stuck open, and unles the engine is worked hard, the coolant does not keep the heat long enough to get round to the heater matrix, as it would get cooled by the radiator 1st.
just a few thoughts