A3 temperature

Hi,
My A3 1.8 (non-turbo) from '99 seems to have some temperature problems. It started with running very poorly and lacking power when cold (especially
when not driven for more than a day). The dealer said to have repaired this by changing something in the computer. After that it ran fine for a few weeks but now the problem re-appeared, with another problem on top of it: while driving through town tonight with a warm engine I saw the temperature dropping from 90 to 60 and going back to 90 again, within 3 minutes! Also when I drive on the motorway around 150 km/h the temperature does not get above 70. I know for sure that my oil and water are ok, so that can't be the problem. What can it be? The sensor, the computer or the temp meter in the dashboard? Wasn't the dealer a bit short-sighted by just adjusting the computer? Any other suggestions?
Thanks, Casper
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this
temperature
problem.
dashboard?
I have noticed the gauge on my 2000 A3 1.6 Sport doing the exact same thing: moving from 60 to 90 and back in a very short period. It never goes to any temperature other than 60 or 90. I can't say I noticed it doing this before a month or so ago, but it doesn't seem to cause any problems.
I have no issues with the engine/power however, so maybe this is a separate issue/problem.
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