car heater blowing cold when it idels

[Wayne Sheppard]

changed thermostat expansion tank and cap new heater matrix can get really good heat when rev car above 2000 rpm but when driving they start to go cold again or if i let it idel rev above 2500/3000 revs heat comes back any ideas thankyou

[micheal balbrig]

As you covered most possible causes,I expect the next suspect may be water pump.

[Wayne Sheppard]

could my radiator be blocked as took it on a run and it overheated