| Sounds like either a bad earth in the circuit or that there's a fault in the tacho that is affecting the temperature gauge.
Try disconnecting the tacho and rev it again to see if it affects the temp gauge. You may need to take the top fairing off, but the tacho connector is a small 3-pin plug behind it.
If it still does the same then it's obviously not the tacho at fault, so it is most likely a bad earth somewhere.
Check all the connectors for the instruments - these are on push-fit connectors that have little latches on them to lock into place. It's quite easy to not connect them properly so disconnect each one and check the state of the connectors, then refit.
Failing that, it may be a bad voltage regulator? Have you tried the same thing with the lights on and off? | 
antonye
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