As you probably know, there is no choice for FPS when creating AVCHD DVD 1280x720p, you are given only 23.976 fps. If you aren't satisfied with the results of converting from 50p and 60p to 23.976p because the playback doesn't seem smooth, then you may want to try what I do.

My camcorder video is AVC/h.264 1280x720p 60 fps, and I want to create an AVCHD DVD that is 1280x720, 23.976 fps.

First I produce a file that is AVCHD 1280x720, 23.976 fps. Next I start another project and put the produced file on the timeline. I apply the stabilize effect with a value of 15. Finally I "Create" an AVCHD DVD disc that is 1280x720p, 23.976 fps.

The result is a much smoother playback.

You cannot apply the stabilize effect in the first pass to smooth out playback. All that happens there is your 50/60p video is stabilized before it is converted to 23.976 fps. The results won't be the same as applying the stabilize effect to the video that is already converted to 23.976 fps. The stabilize effect in the second pass is for blending purposes. You can use the stabilize effect in the first pass for actually stabilizing the video, but you still need the stabilize effect in the second pass for blending frames.

Gateway DX4380, AMD A8-5500 Quad Core 3.2GHz with ATI Radeon HD 7560D; 16GB RAM; 1 TB SATA 7200 RPM; Windows 8 Pro 64-bit; PDR11, PDVD12.