Shaking at 90mph
 

When i hit around 90 MPH my car starts to sway badly. i can bearly keep it in the road.. and its scary so i cut back and get blown away.. I mean because of this i lost a race against a damn Integra! Right now my cars bone stock suspension.

I think you need to worry about making your car safe before you worry about modifying anything. Like, check all the suspension components and rubber bushings. Replace them. Make sure your shocks are working properly and not leaking. Make sure your springs are still good.

I've hit 120 before and not been the least bit concerned about keeping it on the road. The Z31 was built for that. Long distance, high speed driving. If you're having a hard time controlling it at 90mph, you need to fix it before you make it faster. DUH!

Probably should part it out now, while it's still worth something. :p

First investment is your suspension. Sounds like it's shot. :(