1997 Buick Century over heating

Its hot but not to where you can't hold it. It's kind of hard to tell whether the fan come one because of it being quiet but I believe it does because of a very faint humming sound I hear when the problem occurs. When I checked my coolant reservoir there are some black sludge looking stuff in it but not much.

No I haven't. It doesn't actually over heat. According to the needle in the car it will get close to it but would then indicate that the temp is normal. No lights or anything have came on. It recently started happening and only 3 times in the past 2 weeks from initial problem.
It's difficult to explain it because when I drive the needle will indicate that it is heating up and come very close to overheating but then (while still driving) will suddenly balance itself out and I won't have any other problem out of it, Even when I turn on my heater. The problem only happens when I begin driving after having sit for a very long period of time giving the engine ample amount of time to cool off(ex: over night or 8+hours)