I used to love my old iS76, but had to sell it when I got the W53
Was the down wind session at SP starboard tack? If yes, then try moving the rear strap completely forward, also move your harness line on forward by 1cm or 2cm. Do this only to the upwind lines, leave the lines well back on the down wind side. So basically there set up different on both tacks.
Also, put both footstraps in the outer holes for SP, it will stop the upright stance and help you get more leverage and speed.
C/W the iS86, the iS76 has more nose rocker, so its pretty normal for it ride with a high nose, especially once you get to 35 plus knot speeds. If you don't like the feel of a high nose, a smaller fin will help get the nose down. The stock Drake 32cm fin was pretty grunty.
The NP RSR has a low clew, try moving your mast track right back. This will stop the clew hitting the deck or water. But it might feel better if you move the footstraps on the down wind side all the way back (will also be faster). The flat water SP conditions should allow this.
If your using it on a speed course, its to have harness lines and footstraps set up different on opposite tacks. It may look odd, but it will allow you to be more efficient on both upwind and down wind tacks
Hope you get to figure it out
I really liked the iS76, and regret selling it. It was a true high wind open water slalom board. Just couldn't justify having a both the W53 & iS76.