If you merely pulled car back to factory specs and didn't reinforce rear pickup points you may have some fatigued metal that allows rear suspension to deflect when under loads at the track. I have seen this too. That might explain feels fine on street but then gets funky on track, but too much rear bar is rarely a good thing for a track car. I run stock nismo rear bar with soft springs in rear to keep power down.
Also are bar mounting points all in stock spec? If something was moved in accident you may be getting strange loading of the bar.
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Grant
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