The system will drain itself when you remove the sender!
Sorry, I couldn't let that one go!
You'll need to drain the radiator, at the very least to remove the sender, and not make a big mess.
If you un-plug the sender, does the gauge still act goofy, or does it stay in one spot?
Another thing it could be is the system is not full of coolant, and there are air pockets in it. The sensor won't read correctly unless it is in contact with the coolant, so if the coolant level is low, it could cause the gauge to fluctuate like that. I only mention this because you say it just started after a flush....
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"