Why does Mercedes 190e heat up when idle but when driven engine cools and no longer blows warm air in winter?

I have a 1991 190e 4 cylinder with the same problem - in cold weather the engine does not warm up. I know that I have a head gasket leak due to oil in coolant. I changed the thermostat but no improvement. I suspect the head gasket leak somehow affects the working of the thermostat or circulation of the coolant.