MP4s - how to correct them, then join them together?

[extent]

I have mp4 videos which were created on an n900 mobile. They are of a hi resolution, but n900 phones are fairly poor when it comes to maintaining the same fps when recording. I use corel moviefactory to create dvds, and I can add these mp4s to it easily. This usually works reasonably well, on the final dvd- even more frames are lost as a result of these videos being transferred to dvd, but nothing can really be done in correcting that... and its still fairly watchable.

My problem is that one of my mp4s is a mere 4 seconds long- this is correct, but all pc players state that its 16 seconds when I play it! How can I correct this? I have tried joining the file in both ImTOO and Ultra Joiner to another mp4, and saving the file as mp4 again with no changes, but this causes either lots of interference on playback of the joined file for the first couple of seconds, and also at the precise point in the file that the second video starts. my finished file is too "jerky" (frames lost yet again) with half of the video missing.

Any recommendations on how to fix the first mp4 with its incorrect duration showing on most pc players, before I add it to my dvd creation program?

does this also suggest that most joining tools dont join correctly?

All of my files are mp4 which I am adding to my family dvd. This dvd is created with corel. I thought this would be a simple process, and I have also tried other video to dvd creation tools in the past-but it would seem frame rate is the biggest issue with the final dvds videos, when adding any video files. Most of these dvd creation programs do support a wide range of formats, but I do not know if there is a guideline on the best format to use, or if there really is a way to maintain fps.

Is there a way to check my mp4s so that I know that there should be no issues with them before joining? thanks for any help

[Baldrick]

Try remux them(no video reconversion), in for example xmedia recode, open mp4, under format choose mp4 and under the video and audio tab click video copy and audio copy. Add job and Encode.

Or try yamb or mymp4boxgui or mp4muxer, add the mp4 and make a new mp4.

[extent]

I tried mp4muxer, and it was having problems reading the file.
I tried Xmedia recode, that has saved an mp4 for me that now has the correct duration

The only issue is that the new mp4 either has slightly less frames than the original, or jumps around more in playback if you watch closely.. ?!?? I can understand having a lower quality file, but I wish I knew how to maintain all the frames of the original mp4. In this case, the mp4 was set to keep the "original video" and "original audio", so Im not really sure what the problem is with the files I use.

Ill now be looking around for a good mp4 joining program, but there does not seem to be any around that do not cause either interference in the picture, or duration problems (not keeping/joining the whole file). Can anyone recommend an mp4 or video joiner? thanks