I have a 55 Bel Air with a 350/290 HP crate motor and a TH400 trans. The car needs to be driven for about 30 mins in order to be warmed up enough to downshift correctly. If the car isn't completely warmed up and I try to downshift, it won't, or if I try to stay on the gas while getting on the freeway, it will shift back and forth between 2nd and 3rd gear, until I back off the throttle. Any suggestions? I only drive 10 minutes to work, so I rarely drive the car while it's warmed up enough to shift properly.