On Sunday, February 10, 2019 at 8:00:05 PM UTC-5, Scott Calvert wrote:
At 23:47 07 February 2019, Elena Malanushenko wrote:
Interesting video with the testing of plane
https://www.youtube.com/watch?v=vel-qoc0UY4

I would check for loose seals.

Scott has a good point.
I would suggest a few things.
1) Determine if weight and balance is as it is expected to be. A quick check is to level the glider and get the tail weight. Compare this to weight shown on the wt & balance being used. If different, investigate. Sensitive pitch on tow, especially nose up tendency, and pitch sensitivity in glide is consistent with an aft CG condition. Mid range weight in seats should not require full down trim.
2) Carefully look at all seals on the elevator, external and internal. A shifting seal at a particular air pressure, could easily cause pitch change.
3)If the elevator is trimmed with a tab verify smooth progressive function with no slop or binding.
Good luck
UH