Cut Mp4 Question Avidemux

I have a Mp4 file that I'd like to cut a few seconds off the front. I tried Avidemux, but I don't think I'm doing something right as the outputted file doesn't play. I set the A and B markers, then deleted the portion I wanted. When it was set to a B frame, it gave an error message. When it was set to a P frame, it seemed to work, but didn't play at all in Media Player Classic and was very choppy in VLC. Can anyone point out what I'm doing wrong here? Thanks for any help.

Make sure you use Avidemux 2.6 for that purpose. Also, by definition, if you delete a section from an existing file in "Copy" mode, the first frame after the deleted section must be an I-Frame. Otherwise this would unavoidably leave orphaned (undecodable) P-/B-Frames after the deleted section...

Thanks LoRd_MuldeR. I appreciate the help. I found Avidemux 2.6 and managed to cut out the part I wanted. It plays fine in VLC, but won't play in Media Player Classic, which is my preferred player. Is there something I'm doing or not that's causing this to not work right? I set it to an i-frame as suggested. Thanks.