Oil Cooler Problems

I've got an '84 300D with oil cooler problems, oil does not seem to be circulating through it, even after running at high speed for a long while. The oil cooler never gets warm, nor the lines, so I suspect that the thermostat wasn't openning at the right temp, so I replaced it but it still doesn't get oil circulating through it. If anyone has any idea on what is happening or had first hand experience with this sort of problem your help would be greatly appreciated. EWG