My guess (unless you want to post some video) is that your line hand is pumping up and down too much.
Work on the proper mechanics of the cast with the rod hand and minimize the movement of the line hand while hauling by refining the timing of the pull.
The haul does not need to be big, just correct. The line hand should not have to move out of an oblong area less than one foot wide to throw almost or all of a fly line if the cast itself is done right.
Personally I tend to hold the line off to one side and use more side-pull than down pull which seems to help.
Keep at it.
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