Sounds like you're getting some oil in the cooling system. Unless you've put some additives or flush in there recently, the coolant should not be brownish. Possibly the head gasket is developing a small track between the two systems and it may be on it's way to failing. Try flushing the cooling system and a clean refill and watch it day to day, or if you're paranoid about trusting that the situation will not get worse, get it looked at ASAP.
1987 BMW 325
[This message has been edited by jeffsr (edited 03-12-2000).]