Wheelspin at 80

I've done countless WOT tuning runs on a favorite stretch of road.  I warm the car, scope out the road in one direction, turn around, and do a "pull" in 4th gear from 3000 RPM to either 7500 RPM or the point where I don't like something.  I can't use 3rd gear because the tires always break loose at peak torque, not matter how hot it is outside.  On this particular pull, the tires broke loose at peak torque in 4th gear, at 80 mph.  The rear end kicked out, and I jumped out of the gas.

 The log cursor (white vertical line in the pic) is at a sharp knee in the RPM plot. which indicates the start of tire spin.  VSS, or vehicle speed, which is obtained from a front tire, shows 80.5 mph.  This is at medium boost, 1.5 bar (about 22 psi).  WOT at high boost. 2 bar (29 psi) is nearly undrivable