New here. Need help with Scenic 1.9dti Temp gauge and dash

[Drivers61]

Hi.
I'm new here. I have a 1999 1.9dti.

I have been having some problems with the temperature gauge fluctuating widely for some time.
Sometimes when I take my foot off the accelerator, the needle will go up to the top, and when I accelerate, it drops down. I have changed the sender, but it doesn't make any difference. It appears to be doing it more now.

Does anyone have any ideas?

Someone said to me that it might be the Voltage Regulator behind the dash. I don't think this can be changed. Would it be possible to just change the Instrument Cluster for a second hand one. Or are they unique to the car?

Any help would be greatly appreciated.

James