Sounds to me like the system had air trapped in it. Air in the heater core means no heat. At high RPM the water pump would push enough water through to make heat. At idle there wasn't enough water running through the head, it boiled in the top of the engine and you have the result.
The car should probably be checked for effects of over heating. Cracked or warped head, failed head gasket, and so on.
I pays not to ingore problems like this. I let this happen once, and paid for it. Luckily it wasn't a BMW. A cracked head, $800 and a whole saturady later I was running again. I don't want to know what this costs for a BMW.
PS. It's possible the plastic water pump has failed and caused this problem. Fan clutch may be an issue, but also check the electric fan to make sure it works.
97 328iS, Manual, Stock
Last edited by Gkamysz; 11-17-2005 at 10:37 AM.