If you have an R32/R33 rear end you may need to fit a camber kit. They replace the upper (and lower?) arms with adjustable items that allow the IRS to be setup correctly for the Harrison. I know a bloke down here that recently changed his and said it totally transformed the car. His description of how it behaved before the changes wasn't that different to yours! Also speak to Warwick Harrison, he'll have some advice.
Edit: here's an example of the adjustable camber arms on eBay.
REAR UPPER CAMBER ARM NISSAN GTR R32 R33
Nissan S14 S15 R32 R33 R34 SKYLINE REAR LOWER CAMBER