Hot 85 300sd

[Bruce Appel]

I did a search and lots of reading, but didn't find exactly what I am looking for. Today my 85 300sd got hot enough to spew coolant. Very hot outside, running the ac. Under these conditions it generally runs between 80 and the next line up on the guage . It suddenly shot up, not all the way to the red, and started spewing coolant from the overflow tank. It quickly came right back down, but several times while idling the temp would rapidly shoot up and then go down. Let the car sit for an hour, drove it 20 miles home, temp right around 100 or so. The car runs great in every other way, has 225,000 (!) miles on it.
The only things I can think of to make it behave erratically like that would be the thermostat, and/or problems with the auxillary fan coming on when it is suppossed to.Any other places to look? I figure this is a good time to flush/change the coolant, the thermostat, and then check out the auxillary fan to be sure it is coming on when it should.
What else?