Does the coolant look like it has oil in it, or vice versa? If it's just dirty, flush it and get a new radiator cap. A bad cap can cause it to boil since it doesn't hold enough pressure, which raises the boiling point.
If it is oily, it could very well be your head gasket. I had a '93 Cougar that blew one. It leaked exhaust into the cooling system, which then bubbled out of the overflow tank. It soon started leaking enough to intermittently air-lock the system, which made the temperature gauge go all over the place. Then the gasket really blew, and it made a nice cloud of antifreeze smoke. Fun, fun, fun!
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