Rebuilt the C6 in a Mustang recently, but having a problem. When the xsmn is cold, it shifts perfectly. As it gets warm (15-20 mins on the road), it starts to hold its gears longer and longer before upshifting, and eventually will not upshift at all unless you manually move the shift lever and force it to upshift. I have replaced the vac modulator with no improvement. Since it is apparently heat/pressure related, can anyone give me some other ideas to look at here? Thanks!