rmvb to mkv 720p or 1080p, but video file kind of slow motion, why?

converted a file from rmvb to mkv 720p or 1080p file using AVS 7.1, the converted file video is a bit slow motion, the audio is fine, tried playing it on my computer and tv directly through usb or over the network, same result.

i have another mkv file that plays just fine on the computer or tv, so i used the same settings bit/frame rate, etc. to convert the rmvb, but only difference is the audio, the good mkv file shows AC3, but my AVS 7.1 doesn't have that option, so i used MP3 for the audio.

Is my computer system too slow to do the conversion to mkv 720p or 1080p or is my setting in AVS 7.1 not correct? Any help would be greatly appreciated.

Thanks.