temp gauge raises straight to hot

Hi all, I have a commando 383 and having an issue with my temp gauge.

One the thermostat opens it shoots straight to hot. The radiator gets hot so I'm assuming the thermostat works fine. I've replaced the temperature sender but still having the same issue.

Is there a possibility there's a short in the cable somewhere? Worth changing the thermostat? Anyone else got any ideas or had similar issue?

Thanks guys
Author: grimley