Overheat light on, temp. normal ...

[Evangelos Tozakoglou <vangelis@worldnet.att.net>]

Greetings to all,
I have a problem and I am wondering: Should I burn Deutch marks over the
hood or plain dollars will be enough ?

The car: 5kcsT '86, 173K miles.
The problem:
The overheating light started going on, under normal conditions.
The temperature is on the high side (on, or a little after the 1/2 mark).
That only happened three times so far, twice tonight. It was under regular
"stop and go" conditions, not hot weather or anything out of the ordinary. 
All I had to do was switch the engine off, take a look at the coolant level,
scratch head and get back in ...
The AC was off all the time.
Metal radiator about 9 months old.
There is plenty of coolant and the fan seems to be working.
I tried to duplicate the problem by running the car on idle but no luck. 
Which gremlin is responsible for the cooling ?
Any ideas or pointers are greatly appreciated.
Thanks
Vangelis

PS
Somehow, it gives me a problem every time I get it back from a mechanic ...
You think it is trying to tell me something ?