It’s probably the ground lift, but couple things to consider before concluding that:
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Does the same thing happen if you put the sun down (i.e. dawn, dusk, or nighttime)?
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What is your turbulence setting at?
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Does it happen on a weather preset instead of live weather? If so, how about when wind is set to zero?
I have been alternating between a turbulence setting of medium and low depending on the situation because the profound lift and sink like what you describe will still happen with turbulence set to medium. And the fact that it is supposed to be heat-only ground lift and is directly proportional to wind speed is beyond me.
I understand it is difficult to recreate the vast and dynamic variables which culminate to form weather IRL, but at the very least they could reduce this aspect of the sim experience significantly (or remove it entirely for the time being). Right now, the trade off with the turbulence setting is that you can eliminate this “elevator” effect by reducing the turbulence setting, but you no longer get the jolts and shaking of the airplane that add to the realism. Perhaps, rather than a general “turbulence” setting, they could separate some of these factors to give the user more control over the sim experience.