Checked fluids on my '96 965 and noticed the coolant was brown and had unusual smell. I flushed cooling system last October at 142K miles and refilled with Volvo coolant and distilled water. Since then I've driven about 5K miles. Two months ago, I replaced the gaskets on the oil cooler, which required draining much of the cooling system again. The coolant, which I reused, looked very clean at that time. Other vital signs:
* No overheating & engine runs smoothly.
* The motor oil shows no signs of water contamination.
* The ATF, which has never been changed, looks like what people on the Board say their original fluid appears when they drain it. (I was just getting ready to change it when this issue appeared.)
* The radiator in this car is 7 years / 40K miles old.
* The coolant side of the oil cooler showed no obvious signs of damage - I worked on it to correct an oil leak.

Guess it could be the oil cooler, the radiator, or ?? Looking for suggestions on how to trouble-shoot this.
Thanks.
96 Volvo 965
71 Triumph TR6