Hello this is my first post on this website, I hope I'm posting in the right place. I have a 1972 maverick i6 200 with the c4 cruise-o-matic. The engine and tranny were just rebuilt at shops. Anyways I put about 350 miles on the fresh rebuild and everything was working like a charm, shifts were smooth and happening at good times. Yesterday I was on the high way and everything was working fine but once I got off and slowed down to a stop light that's when I noticed that something was wrong. With the transmission in drive it drags out 1st gear, sometimes not even shifting, if I manually put it in second the shift is smooth. But it will not go into 3rd, if I try to put it in drive when I'm going about 40-45 mph it will act as if it's in neutral, no power making it to the wheels. I replaced the vacuum modulator and the hose coming off both ends of the line. At the modulator at idle it holds about 15 in. of vacuum and responds to throttle. What should by my next course of action? Any help is appreciated thank you.