Shifter Vibration and Difficult Shifting
 

The shifter on my '87 RX-7 seems to vibrate a lot and has difficulty shifting at higher RPMs. The higher the RPM, the worse the issue gets. At redline, the shifter shakes very violently and it's extremely hard to shift into the next gear.

I already replaced the shifter bushings and the issue still happens. I should note that I'm missing the return spring, so the shifter doesn't center from 1st/2nd gear. I don't know if that could be causing the problem, but from my research it seems like it's too much trouble to install.

Any help would be appreciated. Thanks.