I'm reminded of an old joke about the South African pilot during WW2 who recounted in the bar being bounced by "a pair of Fokkers"; a British pilot who didn't think that there were any Fokker aircraft in the area inquired "you say these were Fokkers", "Nah", replied the South African "These fokkers were Messerchmidts".
More seriously, I'd have a look at the design of those trim tabs. I once had a project - a 2 seat light aeroplane with a very significant wing drop at the stall, which after a lot of mucking about we determined was a function of trim speed, which of-course is a function of trim tab angle. The aeroplane had trim tabs - mechanically linked on both sides of the elevator. However, it turned out that there was significant freeplay between the two sides, and because the trim linkage was onto one side only, with the trim set to a lower speed, and under air loads, we got signicant asymmetry between the two trim tabs, which translated into 45 degrees plus wing drop at the stall, always in the same direction. We solved it with the introduction of a couple of extra low-friction washers that eliminated about 90% of the freeplay.
G