Besides all the advise given, none seem to catch the far more "dangerous" symptom your cooling system shows: You have a bad thermostat! I doesn't work prober as it should - or at least that is the very first thing I would attend by those symptoms given!

In other words - if your temp seems to not be "stabile"/smooth when engine is at working temp - your thermostat doesn't do it's job correct - it doesn't open/close smoothly and maybe in not long, it will block the passage totally with an over-boiled top as a follow.

When your engine is at working temp (let's say 10 minutes on the road) - try unscrew the coolant cap - how does water react? Is water showing a boiling behaviour? - like in waves? It might sound a bit strange, but please try! If so - you have a bad thermostat - try remove the thermostat (totally) and run car (yes, it's no problem, just unscrew the thermostat housing and remove the thermostat, but let the rubber seal stay for sealing) - does the symptoms change? (they SHOULD!!) -and you should have a far more constant temp - even on a misread temp. gauge.
(This will now let coolant water float freely, thereof your engine will stay very low temp if you drive arround - letting cool air through radiator - so in this case, stay at the garage, give the enigine some rapid idle for heating up again)

I'll bet you a six-pack that your symptoms will disappear if you change your thermostat. (go for a 82 degree celcius - no matter where you are!)


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/Soren