Engine cools down when driving

86hdfxwg

Hi. My pt cruiser cools down when driving and heat blows cold. I've tried 4 different thermostats. New fan relays. Has new head and head gasket. New water pump, timing belt and pullies. If i run heat on low ill get the best heat and engine will stay near normal operating temp. Anything above low on heat setting will drop engine temp cold.
Thanks for any help