Thought it may be related to the lockup not applying fully, so manual turn lockup solenoid on and does it, but only at 80 mph. Turn it on at 65 it locks up, and at 80 it shudders and actually locks the drivetrain up, VSS goes from 80 mph to 73 then climbs back through it ok.
I logged TCC enable status bits, and TCC apply pulsewidth, everythign related to TCC/shift solenoids on this log, VSS, RPM etc to view what I was seeing in these log
Factory Stock 97 SS M6 13.51 @ 104.3 mph
Stock Longblock LS1 w/ 233/238 P.S.I. Cam
10.81 @ 126.9 Full interior, six speed on 275 radials, a decade ago
'99 TA trunk mounted 76mm 6 Liter
9.0s in '09 @ 153 MPH
Turbo 5.3 Volvo 740 Wagon
32psi and still winding out 5th on the highway somewhere
Im not sure, but maybe something is out of balance? Ive seen Fords have a torque converter shudder (infact they have an ATF Additive called Dr. tranny shudder fix for it) but something like this I dont think would be in a tune. Ive been wrong before though. Id check driveshaft and tire weights to start with.
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