Over the past year the 940 with 260,000 mi decides to overheat randomly---typically when in traffic.Yesterday,I checked oil,added 6oz. of water in afternoon and last night it overheated.Let it cool,added fluid and it's running fine today.Could it be the thermostat?

 

Not thermostat. Does it only overheat in traffic?

Regards, Andrew.

 

Yes--only in traffic.

 

I would look at your fan clutch. Get the engine hot, as in running temperature. With engine off, the fan should be VERY hard to turn, almost impossible. If you can grab the blade and turn it with little effort, there's your problem. Also check to see if the fan belt for the waterpump/fan is tight, it should have an inch of deflection in the middle of the longest span, pushed with a finger, or less.
You should also be able to hear the fan clutch kick in as the engine gets to temperature if you idle the car in the driveway with the bonnet up. As the clutch gets to engagement temperature, you'll hear the fan roar as it starts pulling air.

Regards, Andrew.