This may be a wacky answer but could there be varying system voltage somehow? I haven't seen swinging needles in our '84 Spider (Dual Pod dash) but I have seen it in our GTV6. And, I've noticed that the temp gauge in our GTV6 is very sensitive to sysyem voltage. I first noticed this when the alternator died. I was about 60 miles away from home. As the battery voltage dropped the temp gauge also dropped.
Not a wacky answer at all. When I read the symptoms the first time, my initial thought was an electrical issue, possibly related to the wire attached to the sender, but not necessarily limited to just that component.
As an example, consider the ground wire coming off the battery. If the battery itself is not bolted firmly in place, and the bolt securing the battery ground wire to the chassis is not tight, the inertia applied during a sharp right turn might pull the battery to the left and put strain on the cable bolt, and make a better ground connection. Conversely, a sharp left hand turn might cause slack in that same ground cable thereby creating a poorer ground connection during left hand turns.
Since the water temp gauge operates on electrical current, and the readings seem to vary based on differing influences of inertia, why not investigate how changes in the inertia of associated masses could be influencing the electrical connections involved?
Check that the sender's spade terminal is clean, verify that the clip arms on the mating shoe terminal hold it tight, follow the wire and make sure it has enough slack to not be affected when the motor tilts during turns, check related plug connectors, grounds, etc.