Originally Posted by duxthe1

The thermostat is rated for 87C so 120C is definately too hot. I'd be suspicious of the t-stat and radiator. The t-stat may be sticking and is the cheap easy to replace part so you should start by replacing it first. Given the age of the car it is very likely that the radiator is partially corroded and clogged internally. With a known good t-stat you can chek the raditor by feeling for cool spots right after a high speed drive with an immediate shut down.

If the car will cool down when running at high speed then you may have a failing fan clutch that isn't moving enough air when in stop and go traffic.