what climate do you live in? Have you had to top off the coolant recently? Did you add only coolant, instead of 50/50?
you don’t say whether this happens every time, or in different driving condition - city/hiway. so, Even tho it’s summer, try this. Start off and head toward a highway. Have the heat on HI, with the fan just enough ON so you can feel it. Then watch your gauge and air temp as the car warms up. the air should get warm in just a minute, before the gauge starts to rise. The, as the gauge rises, the heat will stabilize at HOT. then, if the gauge falls, what happens to the air temp?
Now, do the same thing from cold start in city driving, or just at idle in the driveway.
The difference might be that at highway speeds, the air blows hotter, but doesn’t at idle. This would mean air pocket in the coolant.
if the air blows the same in both schemes, but the gauge still falls, remove the thermostat, ad drill a small hole thru it, max 1/8". Yhis will allow a small amout of water to pass thru so that the radiator water will be warming up, and when the thermostat opens, the inrush of cold water will not be as severe.
Also, check the coolant bypass line. could be restricted.
cheers