It's probably OK, is this movement new? You might be seeing the thermostat cycle at low / mid speed and the higher speed, higher load is keeping it open.
I have seen low coolant move the gauge around but it is usually more of a dramatic swing.
You could hang a ohm meter on the ( disconnected ) coolant sender and watch for movement.
A trick for determining thermostat opening point.
With an overnight cooling of the engine, start engine and let idle. Watch the temp climb, at some point it will drop a bit due to cold water in the radiator entering the engine, this is the thermostat opening temp. The temp will then climb back up, stat open again but the drop will be less than the first go around. At some point there will be a barely discernible fluctuation as the stat cycles.
This works for most cars however, radiators with a bleed line or heater hose returning to the return side tank ( 80's GM ) the drop in temp during stat opening will be less apparent.
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