The color of milk would indicate oil in the coolant and vice versa. Check the color of the oil itself. If also milky, suspect head gasket. I also had some grunge on my coolant filler cap and found that the water pump soon began to give me vibration headaches (bearing shot and relaesed its grease into the coolant). Replaced the water pump, and coolant filler cap remains clean.
Is the car overheating?
Leaking oil or coolant?
Answers to some or all the ???'s may help in determining the source of the problem, if is a problem at all.