http://www.moviecodec.com/conversion/using-software-to-convert-3gp-to-avi-files-7445/
Try finding a parameter named “fps” or “frames pr second”, in the program you are using for the conversion from 3gp to whatever.
When converting files from my cell phone, this must be set to 12.
[EDITOR’S NOTE: I’m using SUPER to convert a 3GP video from my Samsung cell phone to a standard avi – the above fix doesn’t seem to do the trick, though it worked in the past – Will need to do more research, but I am leaving this post up in case it helps somebody]