Where should the temperature be and why would the reservoir overflow?

[86560SEL]

Normally my 85' MB runs below 80*C on the highway and even sometimes in town if the traffic flow is good. Today was the warmest day we have had since I have owned the car (In the mid 60s) and I had to go a good deal of stop/go driving. I noticed that the temperature gauge had went above 80*C and was actually slightly above horizontal. When I would start back out, it would go back down to slightly above 80*C. When I got back on the highway, it dropped slightly below 80*C. Is this normal?

One thing I am concerned about is that when I made one stop ran in, then back out. The car was probably turned off for about 5 minutes. I came out and saw anti-freeze leaking out under the car. I got home and checked it and it had been leaking out of the overflow tank, as if it was overfull. I let it cool off and checked it a little while ago and now it is at the 1/2 mark. What is going on here? The car was recently driven 300 miles on the freeway with no problems.

Any advice appreciated.