I have the SPL kit that was brackets only, no arms. I thought I would use OEM Toe arms from an NA, but my rear toe was out of alignment with them installed, causing a similar issue you described. The SPL Brackets work great to delete HICAS, but I had to upgrade to some Powertrix Adjustible Rear Toe Arms to actually get rear toe in spec. Once in-spec, no more stability issues over bumps. If you do have adjustable rear toe arms that were recently aligned, I'd be curious what the rear toe spec says on results sheet.