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It's not an easy procedure to get dual elevator servos agreeing on a DX7, even when the linkages are perfect. Any sub-trim necessary to align the arms throws the mix off. Then, you have to juggle the end point and mix percentage numbers (by trial and error) to end up with equal throw and (approximately) equal travel rate. So, yes, any or all of the suggestions above could be the cause, and certainly linkage perfection will make the process easier, and certainly a bum servo will make it impossible. However, it also requires the fiddling with the mix percentage vs. endpoint to get the rise curve as close to the same as possible.

Also, an X9303 2.4Ghz can be had for about $550 for a smart shopper, and is a really, really excellent step up from the DX7.