Problem with Premiere

Very good to everyone. My problem is the following: I was doing a project in the premiere (version CS6) and import 2 different videos to join them. One with better quality than the other. The problem is that when rendering, I played the video and the part that corresponds to the second one is slow, a little stuck. The original video is fine, I verified it and everything I do not know happens. I render it in 720p, 30fps, I already tried 25 fps and 23,9 but nothing. I do not know what to do. Please help.