What tires do you have on the front?
A mismatch front to rear with tires that are either stiffer or softer can cause issues like you describe
2012 KB Boss: MGW Shifter & Knob, FRPP LCAs, Relocation Brackets, Adj Panhard Bar,P Springs, Enkei & Nittos, MM Plates, Girodics rotors, Pagid pads, DSS Driveshaft, Trans
scoop, Brake cooling ducts.
2013 V-6: MT-82, MCA edition, Recaros, Performance package, American Racing classics W/Pilot Sports.