The boost is upsetting the modulator.Read the link,there is a valve made by transgo that is for boost when using a modulator. The boost on the modualtor is causing the no shift and possibly upsetting the modulator enough to affect pressures to cause a flare. Other alternative is to go to manual VB that runs a constant line pressure and does away with the modulator.
http://www.turbobuick.com/forums/transmission-talk/255286-th400-build-pictures-5.html
