Overheating problem - any ideas?

[Fenda]

Anyone got any tips on where to look for cause of overheating. Now got 40,000 on the clock and the temperature gauge is normally rock steady, but riding home yesterday the gauge really climbed up. The coolant collected in the plastic tank and was nearly to to the top (after the bike cooled this went down to normal level).
The overheating occurs only in crawling traffic or even if the bike is left idling for long enough. The fan is working, at least while the bike is idling.
Is there a thermostat I should check?
Any ideas would be appreciated. Cheers,
David
dmateer@bigpond.com.au<i></i>