I have a 85 Elcamino SS with a stock 305 and 3 speed auto transmission. When i go out in the mornings and it's cold outside my transmission doesn't do anything unless i drop it down in low gear and manually shift it. But when it gets warmed up it's fine. I changed the tranny oil and filter back mid summer.Anybody knows whats wrong?

Check the vacuum modulator, it's probbaly old. Evertyhing inside is working fine if it shifts manually. You can pick up one at your local trans shop, the gold ones from Transtar work the best FYI, they're adjustable with a little screw where the vacuum hose goes, it'll help with the shift points.