noticed today that my car started overheating when i pressed the accelerator, then cooled down again when i eased off. An ex mechanic friend of mine said to take off the radiator cap and see what happens when the car is on idle. Well, with a totally cool engine, after about 10 seconds a lot (i mean loads!) of white foam spews out of the radiator, and doesnt stop doing it even with running the car on idle for 3 mins.

panicing a bit about what would cause this?
thanks in advance!