If the anti-freeze is dirty it might be clogging up the radiator and other cooling jackets in the engine. You'll likely need to replace your radiator but flushing your cooling system may help. See Pep Boys on this. Before you go running down there though, pull the dipstick for your engine oil. Inspect it for bubbles, brownish color or a white film. If you have any of these in the engine oil you will need to replace the head gasket. If you are mechanically inclined then it shouldn't be too hard. Tedious, but not too big of a deal. Hopefully, the stuff you see in the reservoir is just lack of coo lent maintenance and flushing the coo lent system will take care of this. Remember to change your anti-freeze at least once a year.