I completely rebuilt my TH350 in my 69 Camaro about 6 months ago (about 500 miles). It worked great after that. However, for tyhe last 3-4 weeks, when I first start it up and shift it into drive, 2nd or 1st gear. I hear the transmission engage, or some sort of clunk type noise in the tranny (but not the driveshaft), and there is no response. The car acts as if it is completely in neutral, even though I am in a forward gear. I rev it up and nothing. If I go to reverse immediately after start up, there is no problem and it goes in reverse and backs up. After a few minutes of warming up, the transmission will then go into drive and act perfectly fine. This is happening all the time. As long as the tranny is warm, no problems. The second it cools down, then only forward gears do not work. There is plenty of fluid. What do I need to look at? All help is appreciated.