What do you mean problems with 5th and 6th gear?....please elaborate!....because like I stated before I had bought a shifter that's looks the just like this one and I couldn't get it in reverse....it did make a huge difference in the throw but I couldn't get in reverse because the shifter wouldn't push down enough to engage reverse here's the link for that shifter
Performance Short Throw Sport Shifter for Nissan 350Z 370Z Infinity G35 G37 | eBay