I just put my new motor in, with same old transmission. The trans was 100% fine when tooking out to do the build. After I got done with my motor I then put everything back in. well I get a base tune done and he changed my shift points. Well I took it down the street but didnt get on it because I was running open headers. So I didnt check to see if my trans was fine or not. I have topped my trans fluid off so I thats fine. well I then take it out the next day and get on it alittle and it shift great in every gear. Then stop by a friends house to show off my car, We drive down the road and get on the highway, Still shift fine. Then drive down the road and stop at auto parts just to look around, leave there to go home, stop at a light and i get on it, once i did i hear something pop, not bad but kinda where i can hear it. let off the gas pull in a parking lot, checked my car didnt see anything, and started driving down the road and thats when it start not shifting into 2nd. Checked my trans fluid, it was fine. Someone said that something could be sticking in the trans. when i put the trans in after the motor was done, alot of fluid came out so i put old fluid with new fluid so i dont no if that had anything to do with it. Well sorry for writing all this, didnt no if it would help find out whats wrong, and thanks for your help